M. Robin on a peculiar Organ found in the Rays. 47 



with our determination. But other results, such as those of 

 Hallstrom 39°-38, Blagden and Gilpin 39°, Hope 39°-5, 

 Deluc 41°, Lef'ebvre Gineau 40°, Dalton 38°, Rumford 38°-8, 

 Muncke 3S°-S04, Stampfer 38°'75, &c., show by their dis- 

 cordance with one another, and their disagreement with our 

 result, the little dependence which can in general be placed 

 on the results of former methods. 



We believe that our new method may be applied with great 

 advantage to a variety of interesting problems. One of the 

 most important of these applications is the determination 

 of the dilatation of glass bulbs by heat, which, though for- 

 merly presenting great j^ractical difficulties, can now be ac- 

 complished in the most simple and decisive manner. The 

 bulb has only to be filled with pure water and reduced suc- 

 cessively to two temperatures, one as much above as the 

 other is below the point of maximum density — the rise of the 

 liquid in the stem of course indicates the contraction of the 

 glass in passing from the higher to the lower temperature. 

 The expansion of the glass bulbs being thus accurately ascer- 

 tained, they may be advantageously applied in determining 

 the dilatations of solutions and other liquids. 



XII. On a peculinr Organ found in the Rays (Raia, Cuv.). 

 ByM. LE Dr. Ch. Robin*. 

 nPHERE exists upon each side of the tail of the Rays an 

 *■ organ which is not mentioned in any of the works which I 

 have hitherto been able to consult. This apparatus however de- 

 serves, on more than one account, to attract the attention of 

 ])hysiologists, and probably of physicists also. 



The two organs united form nearly the third of the entire bulk 

 of the tail of the Rays. The bidk of each, at its largest part, is in 

 one of these fishes of an average size, nearly that of the index-finger. 

 Their origin is towards the union of the first and second quarter 

 of the caudal a])pcndage of the Rays, and they terminate in a point 

 at the extremity of the latter. Their anterior extremity is soft 

 and more or less slender, according to different individuals : it 

 swells gradually as far as the middle of the tail of these fishes ; 

 the volume remains the same as far as the origin of the posterior 

 quarter, whence it diminishes finally to the end. This organ is 

 at fiist almost cylindrical, though a little flattened on the sides 

 (to about the extent of the anterior (piarter) ; in the whole of 

 that part it is enveloped by several thin aiul concentric muscular 

 layers. These muscles soon terminate in as many a])oneurotic 

 layers ; the organ then becomes sul)Cutaneous, and at the same 

 time its form changes, becoming round externally and flattened 

 * Krom the Annals and Miigazinc of Natural History for January 1847. 



