108 Prof. Davy on the presence of Arsenic 



It therefore appeal's to be completely proved that water will 

 not freeze in tubes of very small internal diameter, even when 

 they are shaken, at a temperature very considerably lower than 

 that at which it freezes at once in larger tubes, even when kept 

 perfectly still, as though the actual size exerted a most decided 

 influence in preventing the crystallization of water, which freezing 

 essentially is. In this respect water does not stand alone; for 

 the same retarding action is exerted on the crystallization of salts 

 from solution. This is well seen in the case of bichromate of 

 potash ; for in some cases, when in a very minute state of divi- 

 sion inside the fluid-cavities in other salts, a strong hot solution 

 will not deposit any crystals on cooling, even after having been 

 kept for upwards of a year, but remains as a deep yellow liquid, 

 containing relatively far more of the bichromate in solution than 

 can be retained when the liquid is in larger quantity. Small 

 portions of lava have also often remained as an uncrystalline 

 glass inside the minute cavities in the minerals of volcanic rocks, 

 though it has entirely passed into a crystalline stone when in 

 larger masses. 



XX. On the presence of Arsenic in some Artificial Manures, 

 and its absorption by Plants grown loith such Manures. By 

 Edmund William Davy, A.B., M.B., M.R.I. A., Professor 

 of Agriculture and Agricultural Chemistry to the Royal Dublin 

 Society^. 



IT is well known to chemists that sulphuric acid or oil of 

 vitriol, as it is met with in commerce, almost always contains 

 variable proportions of arsenic ; but it appears to me that this 

 fact has been overlooked by the public, and that they are not 

 aware to what extent this highly poisonous substance occurs in 

 general in commercial sulphuric acid, and thus becomes the 

 means by which arsenic enters the difi'erent substances in whose 

 preparation that acid is employed. 



My attention was first called to this subject by the difficulty 

 I experienced in procuring any commercial sulphuric acid which 

 did not contain a comparatively large proportion of arsenic, ren- 

 dering it quite unfit and dangerous to be used for many purposes 

 of experimental illustration. This arises from the fact, that the 

 vitriol manufacturer has found that it is far more economical for 

 him to make sulphuric acid from iron pyrites (a compound of 

 sulphur and iron), which he can obtain for about twenty-five 

 shillings a ton, than from native sulphur, for which he is obliged 



* Communicated by the Author, having been read before the Royal 

 Dublin Society, April 29, 1869. 



