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XX. Observations on certain Molecular Actions of Crystalline 

 Particles, $c ; and on the Cause of the Fixation of Mercu- 

 rial Vapours in the Daguerreotype Process. By Augustus 

 Waller, M.D.* 



[With a Plate.] 



HEN a piece of glass is covered with a solution con- 

 taining the double phosphate of ammonia and magne- 

 sia, and traces are made upon it by any hard body, it is 

 known that they become visible shortly afterwards by the salt 

 being precipitated upon them. Berzelius, who mentions this 

 test in his Elements of Chemistry, states that Wollaston pro- 

 posed to make use of this fact as a test of the presence of 

 magnesia in solution, which has since been frequently adopted. 

 According to Berzelius, "the cause of this property is of a 

 mechanical nature, probably from the glass being covered with 

 microscopic crystals, the facets of which take a different posi- 

 tion on the traces, for some reason which is not easily ex- 

 plained." More recently, Prof. Liebig has alluded to this 

 subject in his Vegetable Physiology, § 157. These effects 

 are referred by him to a state of unstable equilibrium of the 

 various particles which compose the liquid, which is destroyed 

 whenever a dynamical action is created sufficiently powerful 

 to overcome the feeble attractions, or the inertia of the mole- 

 cules in solution. He ascribes to the same cause the sudden 

 solidification of water, which had remained liquid when below 

 the freezing-point, upon being agitated ; the precipitation of a 

 mixture of potash and tartaric acid ; also the detonation of 

 fulminating powder from the contact of any solid body. 

 Neither of these eminent observers mentions having submitted 

 these traces to microscopic examination, although that is the 

 only manner to test the hypothesis advanced by Berzelius. 



On the present occasion it is my intention to describe some 

 observations I have made, in order to elucidate the influence 

 of molecular action on the precipitation of saline bodies, si- 

 milar to that observed in the double phosphate, and to show 

 that a similar influence is exerted over bodies in a gaseous 

 state and in a state of vapour, and afterwards to point out 

 some phaenomena hitherto unexplained, such as the fixation 

 of the mercurial vapours in the Daguerreotype for instance, 

 which evidently depend upon a like cause. 



In order to obtain the double phosphate, I have generally 

 used a solution containing about ten grains of phosphate of 

 soda with about three of carbonate of ammonia in an ounce 

 and a half of water. 1 have preferred this mixture, because 

 the ingredients are more easily procured, and are less acted 

 * Communicated by the Author. 



