160 THEIR COMPARATIVE EFFECTS. 



duce any marked effect. The slight increase in each case might 

 either have disappeared altogether, or have appeared greater 

 had duplicate experiments been tried in each case. 



The absence of such duplicate experiments also renders less 

 satisfactory the large increase shown in the last experiment upon 

 oats, in which the comparative effects of equal weights of car- 

 bonate of soda and carbonate of potash were observed under the 

 same circumstances. The result, however, is sufficiently favour- 

 able to justify the repetition of the experiment, with the greater 

 accuracy which our present experience enables us to attain. 



But, in regard to the comparative effects of these two carbo- 

 nates, as shown by Mr Main's experiment upon oats, this im- 

 portant remark is to be made, that the application of equal 

 weights of the two substances by no means tests their relative 

 efficiency upon any crop, or in any circumstances. 



Carbonate of soda, as we have seen in the preceding section, 

 contains 62 per cent of water, and only 22 per cent of soda, 

 while pearl ash contains naturally no water, and 68 per cent of 

 potash. But the pearl ash of commerce is usually more or less 

 moist. Suppose it, therefore, to have contained even as much 

 as 20 per cent of water, the respective quantities of the dry 

 carbonates applied in the two hundredweights of each salt were 

 as follows : 



Dry carbonate of soda, . . . . 85 Ib. 

 Dry carbonate of potash, . . . . 179 Ib. 



The experiment merely says, therefore, that 85 Ib. of car- 

 bonate of soda do not produce an equal effect with 179 Ib. of 

 carbonate of potash. Now, according to the table of equivalent 

 quantities given in a preceding chapter,* 87 pounds of dry 

 carbonate of potash ought to produce an equal chemical effect 

 with 67 of dry, or 179 of crystallised carbonate of soda ; there- 

 fore, the 179 Ib. of the former which were applied in this ex- 

 periment are equal in chemical effect to 370 Ib. of crystallised 

 carbonate of soda, while only 224 Ib. were employed. Thus the 

 experiment of Mr Main does not necessarily indicate a more 

 favourable or more powerful action on the part of the potash 

 * Chapter V., 8, p. 99. 



