shadow of the object being exactly reproduced in the colored picture. Natural objects, therefore, of convenient form, such as leaves, may be photographed directly; reproductions from camera pictures require glass positives, or positive paper prints made transparent in the usual way with vaseline. A second process is based upon the peculiar properties of the diazo derivatives of the coal-tar bases, in which the light plays a constructive part in the development of a colored picture. When the diazo compounds are treated with an alkaline bisulphite, they are converted into the diazo sulphonates, on which the action of light is to set free the diazo group from its combinations, but which do not react with phenols and amines. The mixture of a diazo sulphonate with the latter is unattended by any color reaction; but, on exposure to light, the diazo group being set free in presence of a phenol, the development of an azo-color takes place with equal step. In the process based on this reaction, the photographic surface is a mixture of a diazo sulphonate with the alkali compound of a phenol applied to any suitable material. On exposure to light under a transparency, development of color takes place in proportion to the quantity of light transmitted, giving, therefore, a reversed reproduction, or negative picture. When printed, the unattached mixture is dissolved away by copious washing, and leaves the picture, already developed in the azo-color, which is relatively insoluble, permanently fixed upon the fabric or material. The primuline process is simple. It can be practiced with the minimum of apparatus, requires no technical training, and the results are striking and pleasing.
Processes for sterilizing Milk.—The report of Messrs. W. T. Sedgwick and John L. Batchelder, Jr., concerning the milk-supply of Boston, shows that milk drawn directly from the healthy cow is ordinarily free from bacteria, or sterile. It is, however, so rapidly contaminated in the act of milking, and is itself so favorable a medium for the growth of bacteria, that even "pure country" milk contains hundreds of bacteria per teaspoonful. The time required before this can be distributed in the city is so great that milk arriving by rail in Boston contains about 300,000 per teaspoonful, while that taken from wagons or sold in groceries is older and shows from one to ten millions. Mrs. Ellen H. Richards and Mrs. Mary Hinman Abel, who have made an especial investigation of the subject for Mr. Edward Atkinson, find the conclusions forced upon them that a large percentage of milk in daily use is liable to contain disease germs which may under favorable circumstances be communicated to the consumer; and that even healthy milk is a highly putrescible substance, which in its raw state offers a most favorable medium for the culture of many kinds of bacteria that grow in numbers and rapidity, depending principally on the surrounding temperature, and that in the digestive tract, especially of young children, in warm weather this partly decomposed milk leads often to fatal results. Various chemicals have been used to neutralize the acids resulting from the activity of these bacteria, but they have one and all been condemned as injuring the milk or as deleterious to the stomach. It is at present agreed on all hands that only by the application of heat can all this germ life be destroyed and the milk made safe without injuring its food value; and numberless experiments have been made to determine how high a degree of heat must be employed and how long it must be continued. This process is known as sterilization. By the ordinary methods in use considerable changes are wrought in the milk by sterilization; and means have been sought to destroy the bacteria, if possible, at a temperature that would leave the milk unchanged in odor, taste, and appearance. Several processes for this purpose are mentioned in Mr. Atkinson's paper; and it has been found that the object is accomplished by restricting the temperature to 140° Fahr.
Observatory Work at Harvard.—The Director of the Astronomical Observatory of Harvard College calls attention in his annual report to the need of a fire-proof building for the storage of photographic plates. The observatory has received in the last year about nine thousand such plates—some taken in Peru, some in California, and some in Cambridge, and it has in all about twenty-seven thousand of them. They represent the entire sky from the north to