tion has taken place. The decidua does not obstruct the passage of spermatozoa in the early months. Another explanation of superfœtation is the existence of a double uterus. The chief difficulty in the matter, however, is that during pregnancy there are no ovules to impregnate. Scanzonia at one time held that ovulation went on after impregnation had taken place, but he has now changed his opinion.

Dr. Cuthbert had met with a case similar to that related by Dr. Atchison, in which one of the children had died and the other was born healthy. The dead foetus was withered and shrivelled.--American Journal of Medical Sciences.

TRIPLETS.-Mr. Thurton reports (British Medical Journal, May 2, 1868), a curious case of this. The subject was thirtysix years of age. She had a child fourteen years ago, and had not subsequently been pregnant until the present time. February 18, 1868, at two o'clock A. M., was delivered of a living male child, after having been ten hours in labor. The placenta came away in due course, and the mother was comfortable. When the husband came home to his dinner, about twelve hours after the birth of the child, he found that, during the temporary absence of her attendants, she had just given birth to two lively girls. Mr. T. was sent for, and on his arrival, found that the placenta had been expelled; one five or ten minutes after the other. Although the after-birth followed soon after the first born child, and there was the long interval between that and the birth of the others, the patient had comparatively little hemorrhage, and has made an excellent recovery.-Ibid.

MISCELLANEOUS.

HEAT RAYS.-We copy from the Chemical News, the following extract from a lecture by Prof. Tyndall, on account of its exceeding interest:

I have had occasion to say to you once or twice in these lectures that no body in nature is absolutely cold. All bodies are more or less hot. Even ice itself is a hot body compared with solid carbonic acid. In fact, ice would be quite competent to make a mixture of solid carbonic acid and ether boil, it being hot in comparison. All bodies are warm, and all bodies are emitting rays of heat. Here is a platinum wire in front of the

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table, such as we have already operated upon. At the present time that platinum wire is emitting rays of heat of a perfectly definite character. If I connect this wire with our battery you will observe our old experiment. You see the wire is heated to redness; it emits rays of heat, and also to some extent rays of light. Before the electric current passes the wire emits rays of heat which are incompetent to excite vision; but when I raise the temperature of the wire thus, by sending the electric current through it, what becomes of the old rays of heat which it emitted in this invisible state? They still maintain themselves and they become much stronger, but they are still obscure. We mix, with the luminous rays of that wire, the obscure radiation that issued from it before the current made it incandescent. I go on shortening the wire, as in an experiment we made in an early portion of these lectures, we find it gets brighter and brighter, but the rays it emitted before it became red hot at all, are still mingled with the visible radiation. They exist, but they exist greatly intensified; so that the rays which issued from that wire before it came incandescent, are present, as well as the visible rays, but they are raised to a thousand times the intensity which they first possessed. They are still obscure, and have no power to excite vision, but they are, nevertheless, there with a thousand-fold their first intensity. Now I must try to separate before you these luminous rays from the obscure rays; and I must endeavor to operate upon the obscure rays so as to show you some effects that they can produce. I think you will understand the process by which this can be done. I have here a small concave mirror, and this I will place behind the electric lamp. We shall have an image of the carbon points of the lamp produced in that way, and I will throw that image upon the screen. We have now thrown upon the screen an image of the carbon points, whence issues the electric light. If I take another mirror, and converge the rays by it, I can give you a larger image, which, perhaps, will be better seen. Here is now a larger image of the carbon points produced in that way. The image is inverted. You see a considerable amount of light there, but Mr. Cottrell will now fill a vessel with an opaque liquid. The liquid which we used to obtain the opaque solution is called bisulphide of carbon: it is perfectly transparent; and here is the substance called iodine-very well known to many people. This bisulphide of carbon dissolves the iodine with great freedom, and the consequence is the production of this dark liquid, which is so wonderfully opaque that it would cut off the light of the sun at noonday. Strange to say, it is the quality and property of this wonderful substance to entirely cut

away the luminous or visible rays upon which depend the colors you saw on the screen, whereas it allows all the rays of heat to pass through. The liquid is opaque to light, but perfectly transparent to radient heat. Mr. Chapman will place a lens in front of the electric lamp; and thus we obtain this beautiful convergent beam or cone of light tracking its way through the dust of the room towards the thermo-electric light. That will cut off bodily all the light, but still the spot where the pile will be placed will remain very hot. [The cell and pile were then placed in position]. You see that all the light is cut away; but you observe that the needle at once marches away, thus proving that although the light is cut off, the heat rays are left behind.

I want now to try and make these heat rays more evident to you still, and for that purpose I have placed within this camera an electric lamp similar to what I have just used; and behind the electric lamp I have placed a silvered mirror. This mirror will reflect the rays of light from the electric lamp, and will cause them to issue through this window which you see in front. This window is formed of rock salt. Rock salt is exceedingly transparent to the rays of the heat, and also to rays of light; and it is for that reason that I used that substance. I now obtain a convergent beam from the electric lamp. You see a brilliant cone of rays. Mr. Cottrell will now place the opaque solution in front. There it is cutting off all the light, so that you can see nothing. But now I bring this piece of platinum opposite the dark liquid; and observe what occurs. The platinum is raised to a red heat, in perfectly dark air. If, instead of platinum, I take some dry paper, and hold it to the focus of the dark rays, you see that I can ignite that paper. The paper is set on fire. This ignition is caused by the invisible rays of the heat issuing from the electic lamp. I now take a thick piece of metal, and hold it in the dark rays of heat: you see it is melted by the radient heat, and drops down in a liquid state. I will now burn a piece of zinc here. There, you see the zinc is actually set on fire in a place where there was perfect darkness. The air where this zinc is set on fire is perfectly unwarmed. Nothing would be easier than to ignite a cigar in this way in perfect darkness. For instance, here is one which I will ignite. You see it is instantly set alight in a place where there is no light. You might put your eye where that platinum was raised to red heat. I have cautiously approached my eye to that burning focus that you saw there, and allow the rays bodily to enter the eye, and could neither see light nor feel heat. The retina was perfectly dead to those very powerful rays. Sometimes we

obtain the combustion of magnesium by these rays. Here you see we have that beautiful metal set on fire in a place where there was no light whatever-a space of utter darkness. I might set London on fire by means of these dark rays. I have here a glass jar containing oxygen gas, and into this jar I dip a piece of charcoal. I now bring the charcoal into the focus of the invisible rays of heat, and you see the charcoal is ignited by these dark rays, and burns brilliantly in this gas.

I want now to make one or two more experiments in connection with this subject. For this purpose I will take the same mirror which I have just used, and employ another camera which is at the end of the table. The mirror will be placed behind the light, and will reflect a beam of light along the table. Instead of allowing this beam to fall upon the audience and annoying you, I will catch it upon another mirror just as I caught the ray of light by the mirror near the ceiling in an experiment early in the lecture. I dare say many of you see the intense reflection here. There is a focus which would burn your fingers most fearfully if you put them there. I dare say we shall be able to inflame paper at that focus. There you see the paper set instantly in a blaze; and this blaze is produced not by the luminous rays, but by the dark ones. You might put a sensitive thermometer there, and have no result. It is only when the heat falls upon this paper that the heat is produced. We e can burn zinc here as I did in the dark rays. You see the zinc is set on fire, and blazes up almost like a piece of paper. Here is a small vessel containing water, and I will place that in the focus of the rays. I now place another vessel of water in such a way that the light has to pass through it. This will intercept the dark rays which give the heat, though it does not sensibly interrupt the rays of light. At the present time the focus of the rays falls upon the former vessel of water, without any effect whatever being produced upon it. I will now withdraw the vessel of water through which the beam passes before it reaches the mirror, and so allow the heat rays to pass, and you see the water in the vessel as the focus of the rays immediately begins to boil. After a time this water will be thrown into a state of violent ebullition. It is already boiling. This action is due not to the rays of light, but entirely to the dark invisible rays of heat of which I have been speaking.

I make these experiments for the purpose of bringing home to your minds the fact that we owe all our rivers, all our glaciers, and all our snow, entirely, or almost entirely, to these dark rays. The luminous or bright rays of the sun fall upon the tropical ocean, and pierce it to great depths: they are not absorbed;

ut the non-luminous rays-the heat rays of the sun-strike upon the tropical ocean, and they are absorbed very near its surface. It was by the absorption of the dark rays that the water was boiled in the last experiment. These dark rays of the sun which strike upon the tropical ocean, and are then absorbed, heat the surface of the ocean, and thus it is that all the moisture or evaporation is produced.

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And, now, I am sorry to say, we have come to the end of our task. I told you in the beginning that I wished very much to transfer the task of giving these lectures to somebody else, as I was so occupied that I could not make them what I wished to make them, and still I am not sorry that I undertook them. am glad that I have come here, for it has given me great pleaspleasure to meet you from day to day. You have made up by your attention for my defects in lecturing; and I have only to add that I thank you for that attention, and wish you from my heart a happy new year.

A NEW THEORY OF CRETINISM.-Dr. M. A. Netter, principal physician to the Military Hospital of Briancon, communicates to the Gazette Medicale de Strasbourg, (1868, No. 7, p. 77), an article calling attention to a novel theory of cretinism announced by M. le Docteur Chabran, a physician of Briancon. The primary cause of goitre and of endemic cretinism, is not referred to the nature of the waters, the humidity of the valleys, or the excessive fatigue undergone by the inhabitants of mountainous countries; but it is said to be essentially dependent upon the enormous differences of temperature, which, in summer, occur in the high Alps and similar regions, from morning to noon, and from noon to evening. The people carrying goitres, and the cretins, says M. Chabran, are found particularly where in summer, a very elevated temperature ( + 45° in the sun, for example) alternates each day with a very low temperature morning and evening. The less these differences of temperature occur in one locality or another, the less we find of goitre and cretinism. If, as is said, regions altogether elevated are spared the epidemic, it is because that, there, the differences of temperature oscillate within more confined limits. The answer to the question, "how such oppositions in the exterior temperature can produce goitre, the first degree of cretinism?" is given as follows:

Among the functions of our organism, is that of caloricity, a