134 Review of Dr. Thomson 



book, least by stating prior researches, he should stultify his 

 own. 



Under silicon, he furnishes an amusing example of the facility 

 with which mineral analyses may be twisted into any shape that 

 a theorist shall fancy. Thus of nepheline, he says, " Let us sup- 

 pose that in this mineral, every atom of alumina is combined 

 with an atom of silica ; and every atom of soda with an atom and 

 a half of silica." Again under dioptase, " Let us calculate the 

 constituents of this mineral on the supposition that it is a hy- 

 drated sesquisilicate of copper." Knebelite consists, according 

 to him, of silicate of iron, silicate of manganese, and trisilicate 

 of manganese. 



How imperfectly the Doctor is acquainted with the chemical 

 habitudes of saline bodies, on which their mutual decompositions 

 depend, will appear from his employing sulphate of soda as a 

 reagent to detect the presence of lead. He mixes solutions of 

 phosphate of soda and nitrate of lead, and tries the supernatant 

 liquid, after it has become limpid, as follows : " A drop of this 

 liquid was put into a watch-glass, and mixed with a drop of solu- 

 tion of sulphate of soda. No precipitation or opalescence took 

 place, shewing that the liquid contained no sensible quantity of 

 lead O 



We affirm on the contrary, that a solution may contain a very 

 sensible quantity of lead, though sulphate of soda does not 

 occasion in it either opalescence or precipitation ; a fact which 

 we shall state in detail presently. 



Dr. Thomson gets completely bewildered in his 7th chapter, 

 on the relation between the atomic weights and specific gravities 

 of gaseous bodies. Here we find him describing an arbitrary 

 convention of numbers, as a law of chemical combination. The 

 law is thus enunciated, " The specific gravity is equal to the 

 atomic weight multiplied by half the specific gravity of oxygen 



This is a valuable piece of legislation. Dr. Thomson resolves 

 that half a volume of oxygen, weighing 0.5555, when air=l, 

 shall be regarded as the atomic unity, or 1 ; consequently, the 

 atoms of all gaseous bodies may be represented numerically in 

 reference either to that demi- volume 0.5555, or to the weight 

 = 1. Hence the reduction of weights to volumes or specific gra- 

 vities, is done by multiplying by 0.5555. This instead of being a 

 law of chemical combination, is an annoyance created by the 

 present oxygen scale, from which the hydrogen scheme is free. 

 But this matter has been already discussed. Yet, after all, Dr. 

 Thomson is under a mistake in ascribing to Dr. Prout the merit 

 of that rule for converting the atomic weight of a body into the 



* Attempt , .200. t Ibid. 241. 



