Royal Society. 309 



coU of the wire, which was to be subjected to trial, placed in a jar 

 of water, of which the change of temperature was measured by a very- 

 sensible thermometer immersed in it ; and a galvanometer, to indicate 

 the quantity of electricity sent through the wire, which was estimated 

 by the quantity of water decomposed by that electricity. The con- 

 clusion he draws from the results of his experiments is, that the ca- 

 lorific effects of equal quantities of transmitted electricity are pro- 

 portional to the resistances opposed to its passage, whatever may be 

 the length, thickness, shape, or kind of metal which closes the cir- 

 cuit : and also that, cceteris paribus, these effects are in the duplicate 

 ratio of the quantities of transmitted electricity ; and consequently 

 also in the duplicate ratio of the velocity of transmission. He also 

 infers from his researches, that the heat produced by the combustion 

 of zinc in oxygen is likewise the consequence of resistance to electric 

 conduction. 



The Society then adjourned over the Christmas recess, to meet 

 again on the 7th of Januaiy next. 



January 7, 1841. — The following communication was read, viz. — 



" Variation of the Magnetic Declination, Horizontal Intensity, 

 and Inchnation observed at Milan on the 23rd and 24th December 

 1840." Communicated by Professor Carlini, Director of the Milan 

 Observatory. 



A paper was also read, entitled, " On the Chorda dorsalis." By 

 Martin Barry, M.D., F.R.SS. L. & E. 



The author of this communication, after pointing out the similarity 

 in appearance between an object noticed by him in the mammiferous 

 ovum, and the incipient chorda dorsalis described by preceding ob- 

 servers in the ova of other Vertebrata, mentions some essential dif- 

 ferences between his own observations and those of others as to the 

 nature and mode of origin of these objects, and their relation to sur- 

 rounding parts. Von Baer, the discoverer of the chorda dorsalis, 

 describes this structure as " the axis around which the first parts of 

 the foetus form." Reichert supposes it to be that embryonic struc- 

 ture which serves as " a support and stay " for parts developed in 

 two halves. The author's observations induce him to believe that, 

 instead of being " the axis around tvhich the first parts of the foetus 

 form," the incipient chorda is the last-formed row of cells, which 

 have pushed previously-formed cells farther out, and that, instead of 

 being merely " a support and stay " for parts developed in two 

 halves, the incipient chorda occupies the centre out of which the 

 " two halves " originally proceeded as a single structure, and is it- 

 self in the course of being enlarged by the continued origin of fresh 

 substance in its most internal part. 



The author enters into a minute comparison of the objects in ques- 

 tion ; from which it appears that the incipient chorda is not, as Baer 

 supposed, developed into a globular form at the fore end, but that 

 the linear ])art is a process from the globular ; and that the pellucid 

 cavity contained within the latter — a jjurt of jjrime importance, being 

 the main centre for the origin of new substance — is not mentioned 

 by Von Baer. Farther, that the origin of the " laminje dorsales " 



