1824.] Atomic Weight of Boracic and Tartaric Acids. 245 



we estimate its expansive power ; besides, it is commonly known 

 that tubes of every description, in course of time, become less 

 ductile ; hence it is not improbable, that a change takes place in 

 their rates of expansion. 



Having been at first induced by the highly sanctioned cele- 

 brity of the Memoir to peruse it with a view to establish a 

 proper graduation of the higher part of the thermometric scale, 

 I may, on some future occasion, show, what that graduation 

 ought to be, for the degrees above 212°, and for those below 

 32° ; beyond which two unalterable points, no scale hitherto 

 laid down, gives indications corresponding to those of the 

 degrees within the limits of the primary thermic unit. 



Besides, as it must be granted that an error exists in the 

 lower part of the mercurial thermometer, so must it likewise in 

 that filled with spirit of wine, and probably to a greater extent ; 

 as perhaps it has never been proved, that the expansive powers 

 of alcohol are, for equal increments of heat, similar to those of 

 mercury, particularly at the low temperatures reported by recent 

 navigators, as having been observed in the polar seas. 



Indeed it would be no easy matter, except in climates where 

 very low temperatures prevail, to determine the rates of the 

 spirit thermometer, with reference to the mercurial one ; but to 

 attempt doing this by comparing the two instruments as at pre- 

 sent constructed, would lead only from one erroneous system of 

 graduation to another. 



It will have been perceived that some of the expansions stated 

 above, are greatly less than those given by the best authors. 

 But as the deductions are founded on the supposition that the 



absolute dilatation of mercury is really — -'; either the legitimacy 



of these deductions, as now made, or the number itself, may be 

 called in question, if any credit be due to former determinations, 

 by experimenters of very high reputation. 



James Crichton. 



Article II. 



On the Atomic Weight of Boracic and Tartaric Acids. By 

 Thomas Thomson, MD. FRS. Regius Professor of Chemistry 

 in the University of Glasgow. 



(To the Editor of the Annals of Philosophy.) 



DEAR SIR, 



In the table of the atomic weights of chemical substances 

 which you inserted in the Annals for March, 1824, I perceive 

 that you make the atomic weight of boracic acid 275, and of 



