Chemistry. 18T 
lius has suggested a safety tube, filled to the thickness of two 
inches with circular pieces of fine wire-gauze, which was suc- 
cessfully tried in his presence by M. Barruel. This distinguish- 
ed chemist was not aware that the very same improvement was 
suggested and used by our eminent countryman Dr Hope, who 
exhibited it to the Royal Society of Edinburgh on the 5th of May 
1817. See Journal of the Royal Institution., vol. hi. p. 376. 
18. Purpuric Acid. — In vol. i. p. 209, we have stated, that 
M. Vauquelin denies the existence of the purpuric acid dis- 
covered by Dr Prout. There seems to be no doubt, however, 
that this acid exists, and Dr Prout has ascribed M. Vauquelin’s 
want of success to his having operated upon an impure lithic 
acid. Dr Prout always employed pure lithic acid, prepared 
from the excrements of the Boa Constrictor. In order to con- 
vince M. Vauquelin of his error. Dr Prout has transmitted to 
him a quantity of purpuric acid, purpurate of ammonia, and 
pure lithic acid. 
19. Specific Gravity of the Alloy of Tin and Platinum. — The 
specific gravity of the alloy of tin and platinum, used by one of 
our correspondents who repeated the experiments of Mr Fox 
(see vol. i. p. 416), was found by Mr Jardine to be 9.7 ; that 
of the platinum being 21.0932, and that of the tin 7.308. 
20. Economical Scales for Thermometers. — The late Dr Pa- 
trick Wilson, Professor of Practical Astronomy in the Univer- 
sity of Glasgow, was remarkable for his great skill in the con- 
struction of thermometers. Such experimenters as happen now; 
to possess any of them, value them highly. Many years ago, 
the writer of this notice heard the late Dr Moyse speak in terms 
of great approbation of Dr Wilson’s thermometers ; and men- 
tion as a remarkable fact, that notwithstanding their accurac}^, 
the scales were generally engraved on paper. The Doctor could 
not conjecture by what contrivance the ingenious constructor 
could so accurately fit his instrument to his scale, instead of 
following the common practice of adapting the scale to the in- 
strument. An opportunity afterwards occurred, of asking an 
explanation of the matter from Dr Wilson’s son, then living 
at Hampstead, (but who died about eight or ten years ago.) 
He stated, that his father had a variety of paper scales printed 
