1 823. ] On Hydrate of Chlorine. 8 1 



at a low temperature, as silver. The same care is to be ob- 

 served in hardening : the article is to be brought to a cherry-red 

 colour, and then instantly quenched in the cold fluid. 



In tempering, which is best performed in a metallic bath 

 properly constructed, the bath will .require to be heated for the 

 respective alloys, from about 70 to 100 of Fahrenheit above 

 the point of temperature required for the best cast steel. We 

 would further recommend, that this act of tempering be per- 

 formed twice ; that is, at the usual time before grinding, and 

 again just before the last polish is given to the blade. This 

 second tempering may perhaps appear superfluous, but upon 

 trial its utility will be readily admitted. We were led to adopt 

 the practice by analogy, when considering the process of 

 making and tempering watch-springs. 



On Hydriodide of Carbon *. 



IN the ' Philosophical Transactions 'for 1821, 1 have described 

 a compound of chlorine and olefiant gas f, but had not at that 

 time the means of ascertaining its composition. Since then I 

 have obtained it in greater quantity, and analysed it. Four 

 grains were passed in vapour over heated copper, in a green 

 glass tube ; iodide of copper was formed, and pure olefiant 

 gas evolved, which amounted to 1*37 cubic inch. As 100 cubic 

 inches of olefiant gas weigh about 30' 15 grs., so 1'37 cubic 

 inch will weigh 0'4<13 gr. Now 4 grains minus 0*413 leaves 3*587 

 iodine, and 3-587 : 0-413 : : 117-75 : 13-55 nearly. Now 13*55 

 is so nearly the number of two proportions of olefiant gas, that 

 the substance may be considered as composed of 



1 proportion of Iodine .... 1 1 7*75 



2 proportions of Olefiant gas . . 13*4 



and is therefore analogous in its constitution to the compound 

 of chlorine and olefiant gas, sometimes called chloric ether. 



On Hydrate of Chlorine J. 



IT was generally considered before the year 1810, that chlo- 

 rine gas was condensible by cold into a solid state ; and we 



* Quarterly Journal of Science, xiii. 429. f See page 61. 



% Quarterly Journal of Science, xv, 71 



