38o 



THE POPULAR SCIENCE MONTHLY. 



the proper time a current of electricity is 

 establi>ho(l tlirougli the gas in the recepta- 

 cles, cau.sing an intense violet light, capable 

 of acting clieniically on the paper for a suffi- 

 cient length of time to photograph the t^had- 

 ows of the cotnpass-needle and the mercury 

 column. Witliin three miimtes the opera- 

 tion 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 con- 

 sumed by grain during its growth. He 

 found that corn in pots, filled with earth 

 and watered daily, consunjed 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 hec- 

 tare (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 ma- 

 nure, and vice versa. Some manures may 

 effect a very considerable economy in the 

 mass of water consumed. 



Vegetation as a Disinfeitant. In a paper 

 advocating the utilization of sewage for ag- 

 ricultural 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 ryegrass, but to which or- 

 ganic matter had been added, became putrid 

 in a few days, but no such putridity ap- 

 peared 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 dwin- 

 dled away and died. Thus it appears that 

 living vegetation acts as a powerful disin- 

 fectant, assimilating directly the nitrogen- 

 ous principles of organic substances. 



Nutritive Valae of Cocca. The nutri- 

 tive constituents of cocoa correspond very 

 closely with those of beef, and largely ex- 

 ceed 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, per- 

 haps, 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 po- 

 sition 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, Lt 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 thsn in wheat." 

 The value of cocoa as a food is thus ap- 

 parent, and fully justifies the high eulo- 

 giums which have been passed upon it. 



