198 Composition and Value of Guano. 



apjilication. It may well be, indeed, that every difference in the 

 constitution of a salt will give it a distinct influence on the growth 

 of plants ; but it is far more consistent with the present position 

 of the chemistry of agriculture to refer the value of different che- 

 mical compounds as manure to the proportion which they 

 respectively contain of the known agents of vegetation, making, at 

 the same time, due allowance for the state of solubility and other 

 accessory circumstances. Thus, if it were a question between 

 the carbonate and oxalate of ammonia, we must, with our present 

 knowledge, value them solely by the relative amount of ammonia 

 they contain, setting aside altogether the difference of the acid 

 with which they are combined. 



It often happens that we have to deal with compounds which 

 may be supposed to possess more than one source of importance 

 in an agricultural point of view ; thus, for instance, sulphate of 

 potash is capable of supplying two, in fact three, necessary sub- 

 stances to a growing plant — sulphuric acid, sulphur, and potash. 

 It is plain that in this case we must place a value, not only upon 

 the potash, but upon the sulphuric acid, since it is known that 

 both these constituents form part of the materials with which 

 Nature builds up her vegetable structures. But there is no 

 reason to think that the sulphur or sulphuric acid of sulphate of 

 potash is more easy of acquisition than the sulphuric acid or sul- 

 phur of sulphate of lime. As a source of these substances to 

 plants, the relative value of the two salts is to be decided solely 

 by the quantity they contain. Then, again, we are not prepared 

 by any means to allow that one salt of any given substance, such 

 as ammonia, will be more efficacious as a manure than another 

 compound of the same substance with a different acid ; there are, 

 for instance, no good grounds for believing that sulphate of am- 

 monia is different in its action to sal-ammoniac, otherwise than in 

 containing a different proportion of the important ingredient, the 

 ammonia. But, as before said, there can be no equality between 

 the phosphate and muriate of ammonia, because the former sup- 

 plies two highly important elements of vegetation, the latter only one. 



If {as a provisional doctrine only) it may then be considered 

 safe to value the different chemical substances in reference to 

 vegetation simply by the quantity of the different constituents 

 which they can supply, we shall have made very great advance 

 towards simplifying our views upon these points, and shall much 

 more readily reduce the question to one of practical economy. 

 This doctrine, when applied with the requisite judgment, will, it 

 is conceived, be not very far from the truth, and it is especially 

 useful in estimatingr the value of ffuano. 



Very many of the analyses which are introduced in this paper 

 were made in my laboratory by Mr. Ogston, Mr. Ward, and Mr. 

 Eggar, and as the greater number of them were undertaken with 



