572 



THE POPULAR SCIENCE MONTHLY. 



shadow of the object being exactly repro- 

 duced in the colored picture. Natural ob- 

 jects, therefore, of convenient form, such as 

 leaves, may be photographed directly ; re- 

 productions 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 con- 

 structive part in the development of a col- 

 ored 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 lat- 

 ter is unattended by any color reaction ; but, 

 on exposure to light, the diazo group being 

 set free in presence of a phenol, the de- 

 velopment of an azo-color takes place with 

 equal step. In the process based on this re- 

 action, the photographic surface is a mixture 

 of a diazo sulphonate with the alkali com- 

 pound of a phenol applied to any suitable 

 material. On exposure to light under a trans- 

 parency, development of color takes place in 

 proportion to the quantity of light trans- 

 mitted, giving, therefore, a reversed repro- 

 duction, 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 pro- 

 cess 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 re- 

 port 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 rap- 

 idly 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 teaspoon- 

 ful. 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 con- 

 tain disease germs which may under favor- 

 able 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, depend- 

 ing principally on the surrounding tempera- 

 ture, and that in the digestive tract, espe- 

 cially 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 with- 

 out 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 or- 

 dinary methods in use considerable changes 

 are wrought in the milk by sterilization ; and 

 means have been sought to destroy the bac- 

 teria, if possible, at a temperature that would 

 leave the milk unchanged in odor, taste, and 

 appearance. Several processes for this pur- 

 pose are mentioned in Mr. Atkinson's paper ; 

 and it has been found that the object is ac- 

 complished 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 rep- 

 resent the entire sky from the north to 



