Firing.-Firing should be gradual, and the grate kept completely covered with coal or ashes. The fire should not be more than four or five inches deep unless the pieces of coal are large, in which case the depth may be increased.

The fire-doors and flue-doors should not be opened in order to keep down the steam pressure. This practice not only wastes fuel but is injurious to the boiler, and will not be necessary if the boiler is properly attended to.

Priming or Foaming.-Foaming is a rapid disturbance of the water, in consequence of which it rises in the boiler in the form of spray or foam; it is usually caused by dirty water, presence of oil, etc., the boiler not having been cleaned for some time or not thoroughly cleaned. Foaming may, however, be due to other causes, such as too small a steam space, sudden demand of a great quantity of steam, etc. In case a boiler foams all steam connections should be shut off and the fire dampened by means of a fresh supply of live coal or ashes. These precautions will usually suffice to allow the water to settle, and to enable one to ascertain the true water level. If the glass shows a small amount of water, start the pump or injector, and fill the boiler to a point between the second and third gauge. The boiler may then be blown off to the first gauge by means of the surface blow-off, if one be present, and if not present the regular blow-off valve may be used. This operation being repeated, the impurities are gradually diminished, but care must be taken that the water level does not fall below the top of the flues. The boiler can now be used as before, but in all cases it should be thoroughly cleaned as soon as possible.

Removal of Scale.-Potatoes, about eight or ten in number, are sometimes placed in the boiler after cleaning. Soda or kerosene may also be injected with the feed-water in quantity to be determined by observation. Boiler compounds should be used with caution, and when used should be obtained from a reliable dealer. Too great a quantity of any of the above will be harmful.

Cleaning. The interval during which a boiler requires no cleaning depends upon the quantity and the quality of water evaporated. Under usual conditions, in order to obtain the best results, a boiler should be cleaned every six or eight weeks. If a boiler is to be cleaned it should be allowed to stand until it is partially cooled off. When blown out cold the metal in the interior will usually be found covered with a thick coating of soft deposit, which can easily be scraped off or washed off with a hose and stream of water.

If a boiler be blown off while the metal is at a high temperature, the deposited matter is usually baked and forms a solid and hard coating, increasing rapidly if not carefully removed by the process of chipping.

Boiler Power.-The manner in which the horse-power of a boiler is usually calculated is far from satisfactory, depending rather upon its size than its power of evaporation.

In 1884 the American Society of Mechanical Engineers adopted the following definite standard:

"A horse-power shall be equivalent to an evaporation of thirty pounds of water into dry steam per hour from feedwater at 100° Fahrenheit, and under a pressure of 70 lbs. per square inch above the atmosphere."

Steam-engine.—The engine should be provided with a governor to regulate its speed, a lubricator to oil valve and piston, and a sufficient number of oil cups, so that all bearings may be properly oiled.

Starting the Engine.-Before starting, all bearings should be supplied with oil, and all waste pipes connected with cylinder and steam-chest opened. The engine should then be started slowly, so as to allow the water to escape. A quantity of steam will always condense as it comes in contact with the cold cylinder-walls, in addition to the water already present in the steam-pipe. This water does not pass off as readily as steam, neither can it be compressed to any great extent. Therefore, if more water be present in the cylinder than will fill the clearance space, and this water not be allowed to escape, the piston moving towards the end of its stroke will strike the water, and consequently be compelled to stop. The greater the speed of the piston as it advances, the greater the force with which it strikes the water, resulting in many cases in a broken cylinderhead.

It is well to have a waste-pipe connected to the steam-pipe at a point just above the engine-valve, in order that the water which has collected in the steam pipe may be blown out before opening the steam-valve.

After the engine has been in operation for a minute or two the waste-valves should be closed.

Horse-power.-The horse-power of an engine may be calculated by means of the following formula:

H. P. = horse power;

P: = mean effective pressure in the cylinder;

L = twice the length of the stroke, in feet;

a = area of piston in square inches;

n = number of revolutions per minute.

ON THE PRESERVATION OF MILK AND CREAM
BY HEAT,

By Dr. H. L. RUSSELL, of Wisconsin Experiment Station,
Author of "Dairy Bacteriology".

On account of the innumerable bacteria that gain access to milk during the process of milking, and subsequent to that time, and the rapid increase of the same in this nutritious fluid, this material universally undergoes fermentative changes, the rapidity of which is largely dependent upon the surrounding temperature. To increase the keeping quality of milk, it is necessary to annihilate these bacteria or keep them under influences unfavorable to their growth.

Heat has been found to be the most efficacious agent in preserving milk in its natural condition. It is applied in two ways, viz., 1. Pasteurization, where the milk or cream is heated for a short time (20-30 min.) at a temperature near the coagulating point of the proteid constituents of the milk (150°-160° F.). 2. Sterilization, where the temperature approximates or exceeds the boiling-point and is applied for a longer time.

The object in both cases is to kill the bacteria present in the milk.

Sterilization accomplishes this most successfully, but it changes the proteid compounds so that the milk has an undesirable "cooked" flavor and odor.

This defect is not found in pasteurized milk, and if properly handled, milk treated by this process will remain sweet from 4 to 8 days.

For use in the near future the pasteurized product is, on the whole, the most satisfactory; the sterilized material being best adapted for export purposes.

The essential condition in pasteurization is that the pasteurizing temperature shall exceed the thermal death point (the temperature at which growing bacteria are destroyed) of disease-producing as well as fermentative bacteria. This temperature for most forms is about 140° F., but certain disease organisms like the tubercle germ of tuberculosis is not killed below 149° F. for 30 minutes, or 155° F. for 15 minutes. As this germ is often found in milk from tuberculous cows, prudence dictates the use of this temperature as a standard for the pasteurization of milk and cream. The proteids in the milk are slightly affected at this temperature, but if the milk is thoroughly chilled, the "cooked" flavor disappears.

The application of this temperature kills only the growing bacteria, and does not affect the latent spores. If after being heated the milk is allowed to cool slowly, and is left at a comparatively warm temperature (exceeding 55° F.), these spores germinate and soon change the character of the milk, so that the value of the heating process is lost. To be efficient, it is necessary to rapidly cool the pasteurized product below the germinating point of the spores, for if they are once allowed to sprout, they will develop slowly at a very low temperature.

In pasteurizing milk or cream, the apparatus should be constructed so that a definite quantity of the fluid can be held at any desired temperature for any length of time, and during the process protected from infection from the air. The apparatus must also be made so as to be easily cleaned and thoroughly sterilized by steam throughout. The milk must be protected from air infection during its withdrawal from the pasteurizing vat into storage vessels (cans and

bottles), and should be thoroughly chilled in a refrigerator for several hours (better over night) before being delivered to the consumer. This chilling process should succeed the heating operation as quickly as possible, as the sudden transition in temperature from 155° F. to 55° F. or less has a paralyzing effect on the development of those organisms (spores) that are not killed by the heat. A large number of machines have been put on the market, but they have for the most part been designed primarily from the dairyman's standpoint, and while they fulfill their requirements as to capacity, cheapness, etc., yet they cannot be relied upon to treat the milk in a way so as to free it with certainty from all possible disease-producing bacteria.

Pasteurization in this country is applied with great success to milk and cream where these products are used in the liquid form. It is used to some extent in this country, but much more widely in continental Europe, in the preparation of cream for the manufacture of butter by the use of a pure culture-starter. It can also be used advantageously in the hot months for increasing the length of time that by-products of the factory like skim-milk and whey may be preserved.

Pasteurization, as well as sterilization, reduces the body, consistency, of milk and cream, and these products therefore seem thinner after having been subjected to the process of heating than before. To obviate this, Dr. Babcock and the writer in 1896 recommended the addition of a small quantity of a solution of sucrate of lime (“viscogen") to the milk or cream, which will restore the consistency of the products, and in case of cream, greatly increase its whipping quality. (See Bull. No. 54 or thirteenth report of Wisconsin Experiment Station.)

DIRECTIONS FOR THE STERILIZATION OF MILK

(U. S. Dept. of Agriculture.)

The sterilization of milk for children, now quite extensively practised in order to destroy the injurious germs which it may contain, can be satisfactorily accomplished