ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1037 



near Paris, and designated the ' Metre des Archives,' the length of the bar 

 is said to be one metre. If now the bar is submerged in a liquid which 

 has throughout its entire mass a temperature one degree higher than that 

 of melting ice, its length, after it has reached the same temperature as the 

 liquid, will be increased by a certain fraction of its entire length. If this 

 length is subdivided into one million equal parts, and if the increase is, for 

 example, ten parts in one million, the coefficient of expansion of the metal 

 is said to be ten mikrons. If the increase in length proceeds uniformly 

 for each and for every increment of temperature, we can say, for example, 

 that the length of the bar at 100° C will be 1000 mikrons, or one 

 millimetre greater than it was at 0^ C. We can also say that if the 

 temperature of the entire mass of metal is again reduced to 0° the length of 

 the bar will be exactly the same as it was before the increase of temperature 

 took place. 



There is some evidence that when certain metals are exposed to very 

 violent changes in temperature, as when zinc is removed from a tempera- 

 ture of 100^ C. and is submerged in melting ice, the molecular arrange- 

 ment of the metal is disturbed to such an extent that the return to its 

 original condition may be delayed for several days, and even for several 

 weeks ; but it cannot, at the present time, be positively asserted that the 

 return will not ultimately take place. 



It will be noticed that the definition of the coefficient of expansion 

 which has been given, viz. the increase in length due to an increase of 

 temperature from 0° to 1°, contains the important limitation that the entire 

 mass of the metal shall have reached the temperature of 0^." 



We have no report of the remaining part of the address, except the 

 following abstract of the ' Pittsburg Dispatch,' which gave an account of 

 the proceedings of the meeting : — 



" Prof. Eogers chose as his subject, ' A demonstration of the fact that 

 metals may be safely employed to measure temperature by means of their 

 expansion imder an increase of temperature.' He began with a defence of 

 microscopy as a science, and gave a brief review of the various ways in 

 which the usefulness of the Microscope may be extended, especially in the 

 direction of mechanical constructions. He then proceeded to discuss the 

 Microscope as a factor in the determination of a constant of nature, which 

 •was practically the real subject of his address. In general the problem to 

 be considered is, ' Do metals expand uniformly under every variation of 

 temperature ? ' After limiting the definition of the term ' constant of 

 nature,' to the three bars of metal investigated, viz, a bar of Baily's 

 metal, composed of 16 parts copper, 2 J parts tin, and 1 part zinc ; a 

 bar of Jessup's steel and a bar of glass made by Chance & Sons in 1870 

 for the British Board of Trade, he gave an account of the various kinds of 

 errors to which observations of this class are liable. Incidentally he 

 referred to the different kinds of thermometers in use, and the manner in 

 which they are constructed, relating many interesting experiments showing 

 the real value of their indications, and how they sometimes fail to register 

 correctly on account of atmospheric changes and conditions. After describ- 

 ing the methods employed to detect the errors of the thermometers em- 

 ployed to measure the temperature at which these three standards of length 

 \\ ere compared, he gave an account of the investigation by which he deter- 

 mined that the relative coefficients of expansion of these metals are constant 

 for all temperatures between — 5° and 95° temperature. He made 293 sets 

 of observations, nearly all of them about half an hour after sunrise on 

 clear days, and a little later on cloudy days. The time at which the com- 

 parisons between the lengths of these standards were made, was defined by 



1887. 3 Y 



