John James Waterston to Faraday   20 January 1860¹

26 Royal Crescent Edinburgh | January 20 1860

Sir

In a paper that I am drawing up for the Philosophical Magazine (entitled - “On the Gradient of Density in saturated vapours and its development as a physical relation between bodies of definite chemical constitution ²) - I have made use of your observations on the condensible gasses that appeared many years ago in the Phil: Trans:³ To do so with effect I have to reduce the temperatures to the standard scale of the air thermometer[.]

The following is the extract from your memoir describing the means employed to measure the low temperatures obtained “In order to obtain some idea of this temperature (i.e. the temperature of the liquid carbonic acid and ether bath) I had an alcohol thermometer made, of which the graduation was carried below 32° F by degrees equal in capacity to those between 32° & 212°” ⁴[.]

I have assumed that the length on the scale of this thermometer corresponding to 10° was uniform and that it was found by comparing the reading with a thermometer of mercury between 0° & 32°[.] On this assumption we can reduce the temperatures very precisely to the air thermometer by means of M. Pierres⁵ observations on the expansion of absolute alcohol⁶ - The results exhibit your observations at these extremely low temperatures to be in conformity with the general law of density of saturated vapours[.]

I shall be highly obliged if you can inform me if I am right in assuming that the scale of your thermometer was thus determined or was it formed as usual with mercurial Thermometers viz: by plunging into melting ice and boiling water & dividing the interval into 180 equal parts for degrees[.]

It is perhaps too much to expect you to remember such particulars so far back, but it is of so much importance in establishing a general principle in science that I have ventured to intrude[.]

I am | Sir | Your obedient humble servant | J.J. Waterston

Address: Dr. Faraday | Royal Institution | Albemarle Street | London