Progress of Invention. 315 



both defects are present at once, the two remedies must be employed 



in succession. Estimation of Silex m Wheat. — It has long been 



known that silex is a necessary constituent of the wheat stalk, a 

 deficiency of it leading to a tendency in the wheat to be easily over- 

 thrown by wind or rain ; hence the application of siliceous manures. 

 On the other hand, it has been found that wheat-straw containing 

 even more than the normal amount of silex is liable to the danger 

 of being prostrated. M. Isidore Pierre has enabled us to reconcile 

 these apparent contradictions. The silex is found in very different 

 quantities in the leaves, the knots, and the spaces between the knots 

 — the leaves containing, for a given weight, far the largest quantity. 

 The more luxuriant the leaves, therefore, the greater the amount of 

 silex, but the greater also the weight to be borne by the stalk ; and 

 the less capable it is of bearing it, being hindered from becoming 

 dry on account of the free access of air being prevented. The 

 errors on this subject have, therefore, arisen from estimating the 

 silex as a whole, and not considering by itself the portion found in 

 the stalk. Not that the entire of what is found in the leaves is inef- 

 fective : for a part of the leaf is in the form of a sheath, which adds 

 to the strength of the stem. But this sheath is not proportionately 

 increased when the leaf becomes very luxuriant. Weight, therefore, 

 but not at the same time strength, is added, when the leaves are 

 greatly developed. Hence the advantage sometimes found in 

 thinning the leaves before the ear begins to form. Water- 

 proof Cement. — It has been found that the addition of coal-dust 

 to ordinary cement renders it completely water-proof, and imparts 

 to it great solidity. For this purpose two parts of fine cement, one 

 part coal-dust finely pulverized, and one and a half parts slaked 

 lime, may be used, the whole being brought to a proper consistency 



by the required amount of water. Sensitive Litmus Paper. — We 



have given an extremely sensitive test for acids ; but as litmus 

 paper is, in ordinary circumstances, very convenient, it is desirable, 

 if possible, so to prepare it as that it may be relied upon. This is 

 easily done. It is highly sensitive only when its colouring matter 

 consists of the red principle of the litmus, combined with sub- 

 carbonate of potash. Any substance having a greater affinity for 

 the red principle than the potash will decompose it. Commercial 

 litmus paper often contains the red principle, united with sub- 

 carbonate of lime, instead of subcarbonate of potash, a compound, 

 decomposed with considerable difficulty. To prevent the presence 

 of the calcium compound, the paper should, before the application 

 of the colouring matter, be immersed in a weak solution of 

 hydrochloric acid, which removes the lime, and thus secures the 

 production of a test paper containing only the higjily sensitive 

 compound. 



