BODIES GENERALLY EXISTING AS GASES. 157 



I have often put a pressure of fifty atmospheres into these tubes, and have had 

 no accident or failure (except the one mentioned). With the assistance of Mr. 

 Addams I have tried their strength by a hydrostatic press, and obtained the following 

 results : — A tube having an external diameter of 0-24 of an inch and a thickness of 

 0*0175 of an inch, burst with a pressure of sixty-seven atmospheres, reckoning one 

 atmosphere as 1 5 lb. on the square inch. A tube which had been used, of the shape of 

 fig. 1, its external diameter being 0-225 of an inch, and its thickness about 0'03 of 

 an inch, sustained a pressure of 118 atmospheres without breaking, or any failure of 

 the caps or cement, and was then removed for further use. 



A tube such as I have employed for generating gases under pressure, having an 

 external diameter of 0*6 of an inch, and a thickness of 0035 of an inch, burst at 

 twenty-five atmospheres. 



Having these data, it was easy to select tubes abundantly suflScient in strength to 

 sustain any force which was likely to be exerted within them in any given experi- 

 ment. 



The gauge used to estimate the degree of pressure to which the gas within the con- 

 densing tube was subjected was of the same kind as those formerly described*, being 

 a small tube of glass closed at one end with a cylinder of mercury moving in it. So 

 the expression of ten or twenty atmospheres, means a force which is able to compress 

 a given portion of air into -r^th or Yo^h of its bulk at the pressure of one atmosphere 

 of thirty inches of mercury. These gauges had their graduation marked on them 

 with a black varnish, and also with Indian ink : — there are several of the gases which, 

 when condensed, cause the varnish to liquefy, but then the Indian ink stood. For 

 further precaution, an exact copy of the gauge was taken on paper, to be applied on 

 the outside of the condensing tube. In most cases, when the experiment was over, 

 the pressure was removed from the interior of the apparatus, to ascertain whether 

 the mercury in the gauge would return back to its first or starting-place. 



For the application of cold to these tubes a bath of Thilorier's mixture of solid 

 carbonic acid and ether was used. An earthenware dish of the capacity of four cubic 

 inches or more was fitted into a similar dish somewhat larger, with three or four folds 

 of dry flannel intervening, and then the bath mixture was made in the inner dish. 

 Such a beith will easily continue for twenty or thirty minutes, retaining solid carbonic 

 acid the whole time ; and the glass tubes used would sustain sudden immersion in it 

 without breaking. 



But as my hopes of any success beyond that heretofore obtained depended more 

 upon depression of temperature than on the pressure which I could employ in these 

 tubes, I endeavoured to obtain a still greater degree of cold. There are, in fact, 

 some results producible by cold which no pressure may be able to effect. Thus, 

 solidification has not as yet been conferred on a fluid by any degree of pressure. 

 Again, that beautiful condition which Cagniard de la Tour has made known, and 



* Philosophical Transaction?, 1823, p. 192. 



