196 DIRECTIONS FOR CONDUCTING 



cases, give nearly the amount of carbonate of lime 

 that has been dissolved out by the acid. Small quan- 

 tities of other substances have been dissolved at the 

 same time, which have been mentioned in a previous 

 chapter, as important to the value of the marl; but 

 they are only to be separated by an instructed chemist. 

 Since expensive manures, such as guano, have come 

 into vogue, the temptation to adulterations on the 

 part of dealers is great, and farmers should be cau- 

 tious in their purchases. By two or three simple tests, 

 the comparative value of a substance offered as a 

 guano, maybe ascertained. Table VI. p 106, will be 

 a useful one for reference in such a case. 



1. A weighed portion may be heated for some 

 hours, at a temperature not exceeding that of boiling 

 water. The loss of weight will then indicate the 

 amount of water which the guano contained, and it 

 can be referred with much probability to one of the 

 classes mentioned in Table VI. 



2. This dried portion may be burned, till it has 

 ceased to lose M^eight: the loss is organic matter, and 

 salts of ammonia; if it is greater than the largest 

 quantity mentioned in Table VI., then it is probable 

 that an adulteration has been practised, by mixing 

 some finely-ground organic substance, such as tan- 

 bark. 



3. The residue after burning should be nearly 

 white, not more than about 36 per cent of the whole 

 weight, and should dissolve almost entirely in muriatic 

 acid. If a large portion refuses to dissolve, some solid 

 substance may have been added as an adulteration. 



4. Some solid may also have been added, which 

 would dissolve in acid; and it therefore becomes neces- 

 sary to ascertain if that which has dissolved be really 

 phosphate of lime. This is simply and easily done by 

 adding ammonia, till the acid solution has become 

 alkaline : if phosphate of lime be present, it will im- 



