﻿with Carbon Dioxide in the Solid State, 87 



beautiful and sharply defined needles of solid mercury, 

 resembling fern leaves, of more than one centim. in length ; 

 the whole mass is coherent and forms now a metal cup, of 

 course with thin walls, that may easily be removed from its 

 mould of solid carbon dioxide and maintains itself during 

 some minutes. 



Effect of Gas and Vacuum Screens. — One of the most inter- 

 esting experiments demonstrated by Prof. Dewar at his 

 admirable lectures on liquid air, seems to me the property 

 of a very high vacuum of preventing the access of heat from 

 the surrounding medium to liquids placed inside. Lecturers 

 on low temperatures will not likely have at command such 

 splendid arrangements as Prof. Dewar could dispose of, but 

 the principle of this fact may be illustrated, in a suitable 

 way, with carbon dioxide. I had three glass tubes of 15 

 millim. diameter constructed, and of equal capacity, and 

 according to Prof. Dewar's device. The first, A, was inside 

 an oblong glass bulb, remaining open, and consequently filled 

 with air that could be exchanged, when desired, for another 

 gas; the second, B, had likewise this bulb, but rarefied with a 

 mercurial pump, thus forming a vacuum-jacket ; the third, 0, 

 was surrounded by two such concentric vacuum-jackets. 

 Placing them in the same support, next to one another, I put 

 in each the same amount by weight of a mixture of carbon 

 dioxide with sulphuric ether; it is then soon observed, within 

 the quarter of an. hour, that A, provided with the air-jacket, 

 is covered outside with a layer of hoar frost; B shows only a 

 slight deposit of condensed aqueous vapour from the atmo- 

 sphere; andC remains transparent, having no deposit outside. 

 Thus the influence of convection and radiation with regard to 

 a vacuum are visible even at great distances from the lecturer's 

 table. 



With the first vessel, A, I carried out another experiment 

 I found suggested in the report of Prof. Dewar's lecture, and 

 bearing on the property of different gases transmitting heat, 

 which they do not all possess to the same extent. In one 

 experiment 1 put 3 grammes of compressed carbon dioxide 

 in the inner tube of the vessel A, when air was in the bulb, and 

 collected the gas, given off by evaporation at the ordinary tem- 

 perature, in a glass jar above water. I noted 170 cub. centim. 

 in five minutes. Supplying again the inner tube with the same 

 amount of carbon dioxide and replacing the air by hydrogen, 

 1 could collect again in five minutes 250 cub. centim. This 

 increasing proportion shows the great conductivity and con- 

 vection that hydrogen possesses. On removing the hydrogen 

 and taking carbon dioxide in gaseous form instead of it. 



