The late Dr. Thomas Thomson. 2G3 



It is but lately that I noticed in a paper on sulphate of zinc, published 

 by T)r. Thomson soon after the appearance of his work, that he freely 

 admits the scantiness of detail, and that he accounts for it in a manner 

 which leads, I think, to an answer to the whole question. " I abstained," 

 he says, " from describing the processes which I followed, because I thought 

 them rather too tedious for a work of the nature that I had projected ; 

 and because it was in my power, in a book intended chiefly for my own 

 students, to supply verbally whatever was wanting in the practical part." 

 I have also the impression, from that paper, and from a review of the 

 work itself, notwithstanding some appearances to the contrary, that the 

 results, which appear so perfect in the " First Principles" are not to be 

 understood as the actual results of any one experiment, or even as the 

 mean of several experiments, but rather as results which might fairly be 

 deduced from them ; and which, being in round, as well as more perfect 

 numbers, were more suitable for a school book. Had Dr. Thomson been 

 more explicit in the work itself, he would have been saved much 

 annoyance, and chemists would have known that the experiments he 

 related were undertaken, and described, more as instructions to his pupils 

 than as contributions to the science. 



It is not without considerable difficulty that I have been enabled thus 

 to reconcile the remarks, not always unjust, which have been made upon 

 this work, with the faithfulness which distinguished all the statements of 

 its author ; and I have entered into the subject on the present occasion 

 the more fully, and with the less hesitation, from being convinced that 

 the appearance of failure in a work like this, which demanded greater 

 delicacy of manipulation than I believe him to have possessed, and a 

 keener eye for possible slight inaccuracies, has acted prejudicially and 

 most unjustly upon his reputation in other departments where these 

 qualities are not so essential, and in which he stands pre-eminent. 



I would not be understood as denying the existence of positive errors 

 in the experiments described in the "First Principles." The mode of 

 analysis, as it is there related, where a salt of barytes is decomposed by 

 one containing suljjhuvic acid, is itself liable to objection; and certainly 

 nothing like accuracy can be expected from a similar experiment with a 

 salt of lead. The chapter on the salts of alumina might also be instanced 

 as defective. 



It is to Dalton that we are indebted for the iirst proper application of 

 Symbols to chemical science. With him a circle represented an atom of 

 oxygen — a circle enclosing a dot was an atom of hydrogen — a circle with 

 a line an atom of azote, and so on. Tliomson indeed had long before 

 used initial letters to denote the composition of minerals; as A for 

 alumina, L for lime, S for silica, and A M G for a mineral containing 

 alumina, magnesia, and glucina. These occur in his article " Mineralogy" 

 which appeared in 1798 in the Encyclopjcdia Britannica, but they indi- 

 cated no particular quantity of tlie substances they represented, and arc 

 scarcely worthy of mention in tliis sketch. In the very first paper, how- 



