The Microscope. 261 



tnass 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 ecjual 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 incurment of temperature, we can 

 say, for example, that the length of the bar at 100° C will be 1000 

 mikrons, or one raillimeterjgreater than it was at ° C. We can 

 also say that if the temperature of the entire mass of metal is again 

 reduced to ° 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 

 temperature of 100 ° and is submerged in melting ice, the molecular 

 arrangement of the metal is disturbed to such an extent that the 

 return to its original condition may be delayed for several days, and 

 for several weeks ; but it cannot, at the present time, be positively 

 asserted that the return will not entirely take place. 



It will be noticed that the definition of the coefficient of expan- 

 sion which has been given, viz: the increase to an increase of tem- 

 perature from 0*^ to 1'', contains the important limitations that the 

 entire mass of the metal shall have reached the temperature of 1° . 



OBSERVATIONS ON A NEW DASYDYTES AND A NEW 

 CH^TONOTUS. 



DR. ALFRED C. STOKES. 



'"T^HIRTY-SIX years ago (1851) in the Annals and Magazine of 

 -*- Natural Histonj, Mr. P. H. Gosse published a short diagnosis 

 of a little aquatic animal allied to Choetonotus, but differing so widely 

 from that well-known miscroscopic creature that a new genus was 

 needed for its reception. This genus Mr. Gosse formulated concisely 

 and gave it tho name Dasydytes, from the Greek dasus hairy, and 

 dutes a diver, at the same time describing two species. The diag- 

 noses of all were so extremely concise that I quote them here: 

 '■^Dasydytes. Eyes absent; body furnished with bristle-like hairs; 

 tail simple, truncate." Of the two species the descriptions are: 

 "D. goniathrix. Hairs long, each hair bent with an abnipt angle; 

 neck constricted. Length j^^ inch. — D. antenniger. Hairs short, 

 downy; a pencil of long hairs at each angle of the posterior extremity 



