106 BOARD OF AGRICULTURE. 



Dr. Nichols. I think that is quite an important point. The 

 gray plaster is made gray by a little trace of iron. I bought 

 twelve tons of plaster the past autumn, but I did not buy it 

 until I had made an analysis of it. Some plaster is largely 

 carbonate of lime, which is an adulteration of sulphate of lime. 

 The real, genuine plaster, is sulphate of lime. The gray tinge 

 in plaster is due to iron. I have observed that there is less 

 carbonate of lime in this gray tinged plaster than in the other. 

 The trace of iron in it is not of any very great consequence, 

 but I find that wherever there are traces of iron, there is less 

 of the carbonate ; there are not so many veins of carbonate 

 running through. So that even in buying plaster, I think it is 

 quite important, if we are going to buy a very large quantity, 

 to have it analyzed. 



Question. How is the manurial value of the phosphates 

 obtained from those Southern rocks ? 



Dr. Nichols. These phosphatic deposits at Charleston ana- 

 lyze about sixty-five per cent, of phosphate of lime. I found 

 that the specimens of superphosphate, made from the Charleston 

 phosphates, were poor ; that the powder was not properly acted 

 upon by the acid. Now, if you buy a bag of these phosphates 

 from Charleston, and only half as much sulphuric acid is added 

 to it as it ought to have, the other part is of no more value to 

 you than sand ; it is an entirely inert element, so far as plant- 

 food is concerned, unless it is changed by sulphuric acid ; 

 therefore, the manufacturers of these phosphates at Charleston, 

 S. C, can deceive by not properly manufacturing them. I 

 have found some specimens where one-half of the super- 

 phosphate was wholly inert. You may take that Charleston 

 rock, powder it up and put it on your land, and it will have no 

 more effect than so much sand ; it is perfectly insoluble. But 

 when that powder is acted upon by sulphuric acid, you liberate 

 the phosphoric acid, and then you get a superphosphate that will 

 tell upon your land. I think that fact shows very clearly the 

 value of chemical experiments. You can see what a miserable 

 product can be made from a good material by improper manipula- 

 tion. I am told that they are now grinding up these Charleston 

 phosphates in Connecticut, and using what is called " chamber" 

 acid, — that is, sulphuric acid, before it is concentrated, — 

 to act upon the phosphates ; and I apprehend that they do not 



