strong "anode cells," i. e., the surface next to the gold offers a higher resistance to a battery current than the other surface of the selenium does. The power to generate a current is temporarily weakened by sending a battery current through the cell while exposed to light, in either direction. The current generated by exposure to light is also weakened by warming the cell, unless the cell is arranged for producing current by exposure to heat. The properties of sensitiveness to light and to change of battery power are independent of each other, as I have cells which are sensitive to change of current but absolutely insensitive to light,— their resistance remaining exactly the same whether the cells are in darkness or in sunlight. I also have cells which are sensitive to light, but are unaffected by change of battery power, or by reversing the direction of the current through them.

The sensitiveness to change of battery power is also independent of the sensitiveness to reversal of direction of the current. Among the best "L B cells," some are "anode cells" and others are "cathode cells," while still others are absolutely insensitive to reversal of current, or to the action of light.

Constancy of the resistance. A noticeable point in my cells is the remarkable constancy of the resistance in sunlight. Allowing for differences in the temperature, the currents, and the light, at different times, the resistance of a cell in sunlight will remain practically constant during months of use and experiments, although during that time the treatments received may have varied the resistance in dark hundreds of thousands of ohms,-sometimes carrying it up, and at others carrying it down again, perhaps scores of times, until it is "matured" or reaches the condition in which its resistance becomes constant.

As has already been stated, the sensitiveness of a cell to light is increased by proper usage. This increased sensitiveness is shown, not by a lowered resistance in light, but by an increased resistance in dark. This change in the cells goes on, more or less rapidly, according as it is retarded or favored by the treatment it receives, until a maximum is reached, after which the resistance remains practically constant in both light and dark, and the cell is then "matured" or finished. The resistance in dark may now be 50 or even 100 times as high as when the cell was first made, yet, whenever exposed to sunlight it promptly shows the same resistance that it did in the beginning. The various treatments, and even

accidents, through which it has passed in the meantime, seem not to have stirred its molecular arrangement under the action of light, but to have expended their forces in modifying the positions which the molecules must normally assume in darkness.

Practical applications. There are many peculiarities of action occasionally found, and the causes of such actions are not always discernible. In practice, I have been accustomed to find the peculiarities and weaknesses of each cell by trial, developing its strongest properties and avoiding its weaknesses, until, when the cell is finished, it has a definite and known character, and is fitted for certain uses and a certain line of treatment, which should not be departed from, as it will be at the risk of temporarily disabling it. In consequence of the time and labor expended in making cells, in the small way, testing, repairing damages done during experiments, etc., the cost of the cells now is unavoidably rather high. But if made in a commercial way all this would be reduced to a system, and the cost would be small. I may say here that I do not make cells for sale.

The applications or uses for these cells are almost innumerable, embracing every branch of electrical science, especially telegraphy, telephony and electric lighting, but I refrain from naming them. I may be permitted, however, to lay before you two applications, because they are of such general scientific interest. The first is my

Photometer. The light to be measured is caused to shine upon a photo-electric current-generating cell, and the current thus produced flows through a galvanometric coil in circuit, whose index indicates upon its scale the intensity of the light. The scale may be calibrated by means of standard candles, and the deflections of the index will then give absolute readings showing the candlepower of the light being tested. Or, the current produced by that light and that produced by the standard candle may be compared, according to any of the known ways of arranging and comparing different lights,—the cell being lastly exposed alternately to the two lights, to see if the index gives exactly the same deflection with each light.

This arrangement leaves untouched the old difficulty in photometry, that arising from the different colors of different lights. I propose to obviate that difficulty in the following manner. As is well known, gold transmits the green rays, silver the blue rays, and so on; therefore, a cell faced with gold will be acted

upon by the green rays, one faced with silver by the blue rays, etc. Now if we construct three cells (or any other number), so faced that the three, collectively, will be acted upon by all the colors, and arrange them around the light to be tested, at equal distances therefrom, each cell will produce a current corresponding to the colored rays suited to it, and all together will produce a current corresponding to all the rays emitted by the light, no matter what the proportions of the different colors may be. The three currents may act upon the same index, but each should have its own coil, not only for the sake of being able to join or to isolate their influences upon the index, but also to avoid the resistances of the other cells. If a solid transparent conductor of electricity could be found which could be thick enough for practical use and yet would transmit all the rays perfectly, i. e., transmit white light unchanged, that would be still better. I have not yet found a satisfactory conductor of that kind, but I think the plan stated will answer the same purpose. This portion of my system I have not practically tested, but it appears to me to give good promise of removing the color stumbling-block which has so long defied all efforts to remove it, and I therefore offer it for your consideration.

Photo-electric regulator. My regulator .consists of a currentgenerating cell arranged in front of a light, say, an electric lamp whose light represents the varying strength of the current which supports it. The current produced in the cell by this light flows through an electro-magnetic apparatus by means of which mechanical movement is produced, and this motion is utilized for changing resistances, actuating a valve, rotating brushes, moving switches, levers, or other devices. This has been constructed on a small scale and operates well, and I think it is destined to be largely used, as a most sensitive, simple and perfect regulator for currents, lights, dynamos, motors, etc., etc., whether large or small.

In conclusion, I would say that the investigation of the physical properties of selenium still offers a rare opportunity for making very important discoveries. But candor compels me to add that whoever undertakes the work will find it neither an easy nor a short one. My own experience would enable me to describe to you scores of curious experiments and still more curious and suggestive results, but lack of time prevents my giving more than this very incomplete outline of my discoveries.

RELATION BETWEEN THE

ELECTROMOTIVE

FORCE OF A DANIELL

CELL AND THE STRENGTH OF THE ZINC SULPHATE SOLUTION.

By Prof. H. S. CARHART, N. W. University, Evanston, Ill.

[ABSTRACT.']

THIS investigation was carried out in the physical laboratory of the university in Berlin. The electromotive force was measured by the compensation method of Poggendorff. The two poles of the battery A to be measured are connected with two points on a second circuit containing a battery B of higher electromotive force than A. The resistance between the points is then varied till no current flows through the circuit of battery A. The difference of potential between the two points common to the two circuits is then equal to the electromotive force to be measured. By Ohm's law the product of the resistance between the two points and the current flowing through the main circuit equals the electromotive force of the battery A.

The current was measured by a silver voltameter with pure nitrate of silver. The resistance employed was in Siemens' units. The deposition of silver in a silver cup was continued ten minutes with a current of slightly over one-tenth of an ampère. The cup was then washed with great care, dried in a hot-air chamber, and weighed after cooling, fractions of milligrams being obtained by taking the swing of the pointer.

The cell to be measured consisted of a U tube, the bend being much smaller than the two branches, the form being that employed by Kohlrausch in measuring the resistance of electrolytes. The lower portion of the tube was first filled with a saturated solution of zinc sulphate; copper sulphate was then added to one branch and a percentage solution of zinc sulphate to the other, the surface of separation between the two adjacent solutions being sharp in each case. No perceptible diffusion of the solutions took place during the time required for the measurement of the current.

The following condensed table exhibits the results:

1 Printed in full in Am. Jour. Sci., Nov., 1884.

SENSITIVENESS OF PHOTOGRAPHIC DRY PLATES. By Wм. H. PICKERING, Mass. Inst. of Technology, Boston, Mass.

[ABSTRACT.]

THE object of this research was to determine the sensitiveness of the various kinds of dry-plates on an absolute scale, and to state it in terms of the sensitiveness of pure silver chloride. The chloride is first exposed in a box one meter in length having an aperture 15.8 cm. in diameter at one end. It is exposed under a graduated scale of tissue paper to the light from the blue sky in the zenith, for a definite time, say eight minutes. The plates to be tested are then cut up in narrow strips, and exposed under the scale to a smaller aperture (.05 cm. in diameter) for a shorter