N'otes on Barometer Construction. 45 



to ascertain when mercury is sufficiently chemically pure 

 and mechanically clean, and fortunately very much of the 

 mercury of commerce is found in a state of almost or quite 

 chemical purity. Moreover it is fortunate that if the 

 mercury to be employed contains lead, tin, or other such 

 chemical impurit}^, it is a matter of no great difficulty to com- 

 pletely separate these metals. In the chemical handbooks 

 you will find directions for several methods of treatment in the 

 wet way ; and you will find not infrequently an objection 

 raised against purification by distillation, but nevertheless I 

 venture to state tliat with all ordinary samples of mercury the 

 method of distillation will be found easy and simple. Should 

 the mercury contain traces of gold and silver — no infrequent 

 occurrence in Victoria — in that case the humid methods 

 described in the books would fail to remove these metals, 

 distillation being the only effective mode of doing so. 



First, it is easy to ascertain the purity of a sample of 

 mercury. You warm and dry it ver}^ thoroughly ; then you 

 fold a piece of clean dry writing paper into a cone, having 

 an exceedingly fine opening at the apex. The warm 

 mercury is poured into this cone, and allowed to run out 

 at the fine aperture in a very thin thread or stream, and 

 collected in a perfectly clean white porcelain basin; any 

 fine particles of dirt will adhere to the paper, and are thus 

 removed, and the mercury collected in the basin, if pure or 

 nearly so, will present a perfect mirror surface. But this 

 brilliancy is not of itself a sufficient index of absolute free- 

 dom from base metals. Take half an ounce or less of this 

 mechanically cleaned and warm mercury, and cause it to 

 gyrate in a porcelain dish, also clean and warm; the metal 

 is mobile enough, and a slight shake of the hand will make 

 it circulate freely, when one of two results will happen — 

 the dish will remain unsoiled, the mercury preserving 

 always the spheroidal form and its perfect brilliancy, a 

 certain indication of its freedom from base metallic impuri- 

 ties ; on the other hand, if there are present the slightest 

 traces of lead, tin, &c., the mercury will form a " talus" or 

 queue, with tarnished surface, and will leave a stain or streak 

 where it has passed over the glazed porcelain surface. 



I notice in certain books a statement about the oxidation 

 of mercury at common temperatures, which appears to 

 demand a remark in this place. With impure mercury there 

 is doubtless, even at common temperatures, oxidation — 



