Page:Popular Science Monthly Volume 7.djvu/394

 the proper time a current of electricity is established through the gas in the receptacles, causing an intense violet light, capable of acting chemically on the paper for a sufficient length of time to photograph the shadows of the compass-needle and the mercury column. Within three minutes the operation is complete, and then the apparatus is hoisted and the paper removed.

Absorption of Water by Growing Grain.—M. Marie-Davy has been making some exact measurements of the quantity of water consumed by grain during its growth. He found that corn in pots, filled with earth and watered daily, consumed 1,796 grammes of water daily to produce one gramme of grain. According to this, a yield of thirty hectolitres (eighty bushels) of corn per hectare (two-fifths of an acre) would take up a quantity of water which, along with the water evaporated, forms a greater total than the amount of the average rainfall of Paris. Thus the yield of the land is limited by the amount of water supplied to the fields. M. Marie-Davy, however, points out that the quantity of water necessary to produce a given harvest is by no means absolute, but depends on the amount of useful mineral matters with which the water can be charged. To a certain extent water supplements manure, and vice versa. Some manures may effect a very considerable economy in the mass of water consumed.

Vegetation as a Disinfectant.—In a paper advocating the utilization of sewage for agricultural purposes, Dr. Alfred Carpenter says that, if a certain weight of rye-grass seed be sown in wet sand, without allowing the contact of any water which contains nitrogenous matter, the plants will grow to a certain size, that is, until they have used up all the matter contained in the seed, and then growth is, to a great extent, arrested. This has been shown experimentally by growing rye-grass under glass. All growth has been arrested for want of nourishment. On adding to the water solutions of fresh organic matter (meat-juice), the plant has at once begun to grow, and in a few days has doubled its size, while a test set of plants to which such organic matter has not been added has remained stationary. Another basia and glass cover with sand not containing rye-grass, but to which organic matter had been added, became putrid in a few days, but no such putridity appeared when the rye-grass was growing. A fourth case had put into it an amount of nitrate of ammonia corresponding to the amount estimated to be contained in the meat-juices which were used in the first case; but here the growth of the plant was by no means so luxuriant as when the living nitrogenous matter was added: although a fresh start was made, the plant soon dwindled away and died. Thus it appears that living vegetation acts as a powerful disinfectant, assimilating directly the nitrogenous principles of organic substances.

Nutritive Value of Cocoa.—The nutritive constituents of cocoa correspond very closely with those of beef, and largely exceed those of milk and wheaten flour: hence the importance of this substance as an article of food. In this respect it differs widely from tea and coffee, which are, perhaps, rather condiments and stimulants than foods, or flesh-formers. The following table, drawn up by Mr. John Holm, of the Edinburgh Chemical Society, shows the position of cocoa as compared with three other well-known articles of food:

"Thus," observes Mr. Holm, "although one-half of the weight of cocoa consists of cocoa-butter, it still presents 20 per cent, of albuminoid material, as against 4 per cent, in milk, 20.75 in beef, and 14.6 in wheat. In addition, it contains starch, which is present neither in milk nor beef, but in smaller proportion than in wheat." The value of cocoa as a food is thus apparent, and fully justifies the high eulogiums which have been passed upon it.