:',L' I >i. ( '. < 'In. !-. / i on 



note as an isolated fact that the immersion errors were only about 

 50 per cent, greater in steam than in i-c, \vhm-as the (litlcivnce between 

 the temperatures of the room and the immersed portion of the ther- 

 mometer was about six times as large in the former case as in the 

 latter. 



It may 1)6 only a coincidence but if so it is a very curious one 

 that in the mean results, both in the ice and steam experiments, the 

 errors diminish in geometrical progression as the immersions increase in 

 arithmetical progression. 



An addition of 1 '25 cm. (i.e., one-quarter the length of the " bulb ") 

 lowers the error almost exactly one-half. 



29. Returning to the general question, we see from Table X that 

 the immersion should be at least 10 cm., in addition to the length 

 of the "bulb"; in all, in the Kew thermometers, at least 15 cm. 

 (6 inches). 



If the whole length immersed is less than 6 cm., the error in an ice 

 or steam point experiment is unlikely to be much under - 1 C. ; and 

 until the total immersion exceeds 10 cm., the error is likely to exceed 

 0-01 C. 



In " fixed point " experiments it is usually easy to have an immersion 

 exceeding 15 cm., and the main results dealt with in this paper are, I 

 believe, free from appreciable uncertainty on this ground. 



There are, howeverj other circumstances under which immersion 

 difficulties arise, as for instance in the comparison of mercury and 

 platinum thermometers at high temperatures. 



Unless both the material of the bath and the contained liquid are 

 transparent, we must have the divisions of the mercury thermometer 

 which we have to read emergent. But with ordinary stirring it is also 

 essential, except in a specially protected bath, that the bulb of the 

 mercury thermometer and the spiral of the platinum thermometer 

 should be about the same level. These conditions are often inconsis- 

 tent with sufficient immersion of the platinum thermometer when we 

 are comparing the lower part of the scale of the mercury thermometer. 

 In fact, my attention was first directed to the question of immersion 

 of platinum thermometers through the unexpected result that raising 

 equally the height of K; and a certain mercury thermometer in a bath 

 of molten metal lowered the reading of K 7 most. 



Another case where insufficient immersion is to be feared is in the 

 determination of melting points of metals such as silver. The crucible 

 supplied for use with melting silver at Kew would, if full to the lip, 

 allow an immersion of only 10 cm. in all. As the silver is stirred 

 during part of the experiment, and is too precious a material to splash 

 all over the furnace, the immersion cannot well exceed 9 cm., and may 

 be considerably less. 



It is by no means impossible that insufficient immersion may have 



