BULLETIN OF THE BUSSEY INSTITUTION. 181 
be made in this country more cheaply than they can be imported, 
and that they should be so made directly. It was this impression, 
doubtless, that led Mr. Lawes to write the following letter, which he 
sent to the Massachusetts Society with his gift of superphosphate :— 
ROTHAMSTED, St. ALBANS, March 25, 1873. 
HENRY SALTONSTALL, EsQ., Treasurer Massachusetts Society for Promoting Agri- 
culture. 
Dear Sir, — Dr. Gilbert has requested me to answer your letter of the 
21st of January, and to inform you that, in accordance with your request, I 
have forwarded to you, through Messrs. Saunders & Co., Liverpool, three tons 
of superphosphate of lime. I am very sorry that so much delay has taken 
place in this matter ; but it was found necessary to have the manure packed 
in casks, and these casks caused delay in the order. I have for some time 
ceased to be a manufacturer of commercial manure, and therefore had not 
the same authority as formerly. The manures I have forwarded to you are 
made entirely from Charleston phosphate and sulphuric acid. I have se- 
lected this substance in preference to our own phosphate, in order to prove to 
you that you have one of the best phosphates which the world produces, but 
also one which can yield a commercial superphosphate cheaper than any 
other. Charleston phosphate, as imported into this country, contains from 
57 to 60 per cent of phosphate of lime. When ground and mixed with sul- 
phuric acid (sp. gr. 1.6), in the proportion of 100 phosphate to about 80 acid, 
it yields a product of 30 to 33 phosphate rendered soluble, and not more than 
3 to 4 per cent insoluble. In your country the phosphate can be raised at a 
cost of less than 20s. per ton. The grinding would cost, at the most, 5s. per 
ton; the acid 50s. per ton. In fact, the net cost of the superphosphate, ready 
to pack into bags, ought not be more than 40s. per ton of 2,240 pounds; and 
all the phosphoric acid should be rendered soluble except 2 or 3 per cent. 
As these manures are required for experimental purposes, I beg to offer them 
to the Massachusetts Agricultural Society without charge, and also the fol- 
lowing summary, which may be said to comprise the results of my experience 
and practice in regard to artificial manures for the last thirty years : — 
The only two substances really required in artificial manures are, — 
1st. Nitrogen ; 
2d. Phosphate of lime. 
Nitrogen is useful in three forms, — 
Ist. As nitric acid ; 
2d. As ammonia; 
3d. As organic decomposable matter, yielding ammonia or nitric acid. 
Nitrogen is more valuable in the form of nitric acid than it is as ammonia, 
and ammonia is more valuable than decaying substances yielding it. The 
