THE FIRST THERMOMETERS. 691 



ing point of water, the melting point of it. But neither these cor- 

 rections nor the incidental recognition by the Florentine Academi- 

 cians of the invariability of the melting point of ice diminish the 

 importance of Reaumur's discovery. 



Having discovered a fixed temperature, Reaumur deduced a 

 way of making spirit thermometers that could be compared with 

 one another. If we plunge a glass bulb prolonged into a fine tube 

 and filled with spirit into freezing water, and draw a line marked 

 zero flush with the top of the liquid, then determine the volume 

 occupied by the liquid under these conditions; if we divide the tube 

 into portions, the interior capacity of which represents at the tem- 

 perature of the freezing of water aliquot parts of that volume — 

 hundredths, for example — and number these divisions from the line 

 marked zero; then if, in an experiment, we see the spirit rise to the 

 level of the division marked five, we know that the spirit in the 

 glass has suffered an apparent dilatation of five hundredths between 

 the freezing temperature of water and the temperature of the ex- 

 periment. If we always take care to use spirit of the same quality 

 — and Reaumur prescribed minute rules on this subject — and if we 

 neglect the changes which the variable nature of the glass will in- 

 troduce into the law of dilatation of the thermometric receptacle, 

 we will obtain instruments of a kind that will always mark the same 

 degree when they are equally heated or cooled. 



For two instruments constructed according to the laws laid 

 down by Reaumur to be rigorously comparable, it was essential that 

 they be made of the same glass and filled with the same liquid. If 

 the glass of which they are made has not exactly the same composi- 

 tion and tempering in both, and the alcohol has not the same degree 

 of concentration, they will not agree. In order to diminish these 

 variations, it is convenient to fix all thermometers, whatever they 

 may be made of, so that they shall give the same indications for two 

 fixed temperatures. The point reached by the liquid at the lower 

 of these temperatures is marked on the instrument, and then it is 

 raised to the higher temperature, and the point which it reaches 

 then is marked. The interval is then divided into parts having 

 the same interior volume, and the division is carried out beyond 

 the fixed points. In such thermometers the liquid will stand at the 

 same mark for an equal degree of heat, notwithstanding slight 

 inequalities in the glass and the fluid. 



It was some time before the two fixed temperatures at which the 

 thermometric scale should be marked were determined upon. Da- 

 lence, in 1688, took a mixture of water and ice for the zero, and 

 the melting point of butter as the upper point. Renaldini, in 1694, 

 recommended a mixture of water and ice and the boiling point of 



