Dr. W. Odliug un the Dvctrinc of Equivalents. 37 



1 may just remark, that if the ship moved with an accelerated 

 velocity, the cable would form a curve with the convexity up- 

 wards, and on the other hand, if slic moved with a retarded 

 velocity, the cable would form a curve with the convexity down- 

 wards. 



With a brake-apparatus properly contrived and worked by 

 practised seamen, the sending and pitching of the vessel in 

 moderate weather ought not materially to increase the strain on 

 the cable, and the nearer the brake-apparatus can be made to 

 approach in its action that of (that exquisitely delicate and simple 

 apparatus) the fly-rod and winch when in the hand of a fisher- 

 man, the more perfect it will be. 



Whilst for very great depths it is very proper to consider the 

 means we have, or may find out, of forming efficient cables of 

 small specific gravity, yet, on the other hand, we ought not to 

 forget that in steel we have a material whose modulus of tension 

 is 6700 fathoms, whilst for iron wire the modulus is only 4000 

 fathoms. 



The subject is very interesting, and I could go on much 

 further, but that I fear I have already trespassed too much on 

 your valuable space, and so with hearty good wishes for the 

 success of the undertaking, 



I remain. Gentlemen, 



Your very obedient Servant, 



15 Park Street, Westminster, S.W., WlLLIAM Gravatt. 



June 15, 1858. 



V. Remarks on the Doctrine of Equivalents. 

 Bij William Odling, M.B., F.C.S.* 



THE theory of polyatomic radicals appears to depend for its 

 due appreciation upon a mode of viewing the doctrine of 

 equivalents, suggested in one form by Gcrhardt some ten years 

 ago, and since modified considerably by the researches of chemists 

 belonging to his school. At the present moment, when the 

 polyatomic radical theory is acquiring such great pi'ominence, a 

 re-enunciation of these hitherto somewhat neglected views does 

 not seem uncalled for. I propose briefly to explain these views, 

 to illustrate their bearings upon tlie jjolyatomic radical theorj', 

 and to contrast them, not only with those which prevailed among 

 the older chemists, but also with tliosc maintained by Laurent 

 and Gcrhardt. 



The word equivalent was originally cm})loyed by Wollaston as 

 a substitute for Dalton's word atom. ^Vollaston's notion of 

 equivalency appears to have been derived chiefly from the phseno» 

 * Commimiciitcd by the Author. 



