in the Standard Points of Thermometers. 123 



some slight variation, so as to appear too low upon the scales 

 of those instruments which have been long made ; and it is said 

 that, in such cases, the just indication was again recovered by 

 breaking off the end of the stem, so as to admit atmospheric 

 air." But, as T had observed that the change went on for a 

 time only, after which it ceased, and that it affected thermo- 

 meters sealed with air over the mercury, as well as those 

 with a vacuum, I undertook the following experiments : — 



In September 1848 I made four thermometers having long 

 degrees, — such that yV° might be easily noted, constructed of 

 the same draft of glass tube ; two of these I placed in boiling 

 water, and kept them at that temperature for a week : my 

 object in this was to learn if any change in the form of the 

 bulb would take place from this slow process of annealing, as 

 glass is known to undergo some change from such exposure. 



The four thermometers were now filled with pure mer- 

 cury : two of these were sealed with a vacuum over the mer- 

 cury ; one tube that had been boiled, and the other hot : the 

 other two tubes were sealed with air over their columns, and 

 the freezing points of all were marked on the tubes ; after 

 which they were placed in a window freely exposed to light, 

 where they were left till January 1849 — a space of four 

 months — when they were again placed in melting ice, and 

 the freezing points marked ; they had risen '24°, -24°, -20°, 

 •06° parts of a degree. The whole four thermometers were 

 now placed in boiling water, and kept there for a week, when 

 the freezing points were again observed to have risen respec- 

 tively -48°, -41°, -60°, -45°. 



The instruments were now left exposed to light as at 

 first ; and, in January 1850, the freezing points were again 

 observed, when they were found to have farther risen '12°, 

 •18°, -20°, -13°; and, lastly, they were observed in May 1850, 

 when no change from last observation was notable. 



The whole amount of rising of the freezing point in these 

 four thermometers, after a lapse of eighteen months, is re- 

 spectively -84°, -83°, -90°, '65° ; and these changes may be the 

 full amount that would take place were the instruments ob- 

 served after a greater lapse of time. From my experience, 

 I know that there is a period after which no change takes 



