William Thomson to Faraday   14 January 1850

2 College, Glasgow, | Jan 14, 1850

My dear Sir

I am extremely glad that you have been so much interested by the subject of my last letter¹.

The conjecture which you make regarding the effects of pressure in altering the temperature at wh[ich] any liquid becomes solid, is entirely in accordance with our views. My brother’s² reasoning would lead to the conclusions that the melting point of any solid wh[ich] contracts on becoming fluid would be lowered by pressure, & that the melting point of any solid wh[ich] expands on becoming fluid would be raised by pressure. His investigation would be applicable to all such cases, with but slight modification of the forms of expression in the cases when there is expansion on melting, and, in every case in wh[ich] the latent heat necessary to melt a cubic foot of the solid, and the volume of the liquid into wh[ich] it is melted, are known, his investigation would lead to a knowledge of the actual amount of alteration in the temperature of melting to be expected from any given, not excessively great pressure.

It would be very interesting to experiment on various solids, especially the metals, both simple metals, & alloys such as Rose’s fusible metal³.

It would also be very interesting to experiment upon the effects of pressure on liquid water, above & below the temperature 39.1, of greatest density, in altering its temperature. Regnault finds that, at the temperatures at wh[ich] he experimented, a sudden pressure of 10 atmospheres did not make as much as 1/50 of a centigrade degree of elevation in the temperature⁴. Hence a very delicate thermometer would be necessary, but I do not think it w[oul]d be impossible to actually exhibit the lowering of temperature wh[ich] a sudden pressure applied to water below 39 ought (according to Carnot’s⁵ Theory) to produce. If a very delicate minimum thermometer of any kind could be constructed, it might be put into a vessel much stronger than any transparent vessel could be made, and two hundred, or more, atmospheres of pressure might readily be applied to the water.

I have today made very careful experiments to ascertain the value of a division of my ether thermometer. By taking a range from about 31˚ to about 34˚ Farh. and using an old mercurial thermometer of Crichton’s⁶ with an ivory scale, not at all delicate, as the standard, I find that 71 of my divisions correspond to a degree Fahr. Without a better standard thermometer I cannot get a more accurate estimate; but I think what I mention may be relied on within 1/20 of the whole amount (I am sure 74 must be greater, and 67 less than the number of my divisions which correspond to a true Fahr. degree[)].

I think on the whole the number .25 which I mentioned in my last letter cannot differ by excess or defect by more than 1/10 of the whole amount, or 1/40 of a degree, from the lowering of temperature actually produced by 19 atmospheres.

I remain, Dear Sir, | Your’s most truly | William Thomson

Prof. Faraday