22 



over-burnt clay partly must be ascribed. I say only partly, 

 because Professor Johnston has already shown that, in over- 

 burning, the constituents of clay are rendered less soluble than 

 they are in the natural clay. My experiments fully confirm the 

 Professor's observations. 1 have further found, that over-burnt 

 clay does not absorb so much ammonia from the atmosphere as 

 properly burnt clay, which is easily explained by the diminished 

 porosity, and consequently diminished absorptive power, of such 

 clays. The cause of the failure attending over-burning, then, must 

 be sought 1st, In the mechanical changes clay undergoes in 

 over-burning ; 2d, In the chemical changes which render such 

 clays less soluble j and 3d, In the diminished power of absorbing 

 gases from the atmosphere. 



8. Does moderately burnt clay absorb more ammonia from the 

 atmosphere than clay in its natural state ? 



It will be remembered that many chemists and agricultural 

 writers ascribe the advantages of burnt clay to ammonia, which, 

 according to their views, is absorbed from the atmosphere by it in 

 that state more extensively than when it is unburnt. In order to 

 put this theory to the test, I made the following experiments 

 with 



1. Clay from Huntstile, near Bridgewater, in its natural state, 

 (the same as that used in the above analyses, marked No. I.) 



2. Clay from the same locality moderately burnt (the same as 

 that used in the above analyses, marked No. II.) 



Both portions were moistened with water, and exposed in glass 

 beakers to the atmosphere for a period of two months and twelve 

 days, without, however, renewing the evaporated water. After 

 that period the quantity of ammonia in each sample was determined 

 by combustion with soda lime in the usual manner. The follow- 

 ing are the results : 



(1.) Clay from Huntstile, in its natural state. 239*15 grains, 

 on combustion, furnished 4'94 grs. of bichloride of platinum and 

 ammonium, or 



100 parts of air-dry clay contained 0-240 per cent of ammonia 

 (NH 4 ,0). 



(2.) Clay from Huntstile, moderately burnt. 210-15 grs., on 

 combustion, gave 0-36 of bichloride of platin. and ammonium, or 

 100 parts of air-dry clay contained 0-019 of ammonia (N H^/0). 



The clay, when unburnt, it will be observed, furnished a much 

 larger quantity of ammonia the same after moderate calcination. 

 We must, however, not conclude that unburnt clay possesses a 

 greater power of absorbing ammonia from the atmosphere ; for the 

 ammonia contained in the analysis is partly the result of the 

 decomposition of nitrogenised organic matters which existed in 

 the clay, and which were destroyed on burning. 



