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1823.] at a low temperature, as silver. The same care is to be observed 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 performed 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.

N the 'Philosophical Transactions' for 1821, I have described a compound of chlorine and olefiant gas, 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 0.413 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

and is therefore analogous in its constitution to the compound of chlorine and olefiant gas, sometimes called chloric ether.

On Hydrate of Chlorine.

t was generally considered before the year 1810, that chlorine gas was condensible by cold into a solid state; and we