DR HOPE ON THE MAXIMUM DENSITY OF SEA-WATER. 243 



When it is considered that a general law has been deduced from, and much 

 reasoning on natural phenomena founded upon, the soUtary experiment, it is a 

 matter of surprise that no one thought it necessary to investigate the matter more 

 fully than Sir Charles Blagden had done, either at the period of the pubhcation 

 of his paper in the year 1788, or during the succeeding thirty-one years. 



Dr Marcet, in his memoir on the specific gravity and temperature of sea- 

 waters, in different parts of the ocean, &;c.. Philosophical Transactions for 1819, 

 records some experiments relative to this matter. He operated in two modes — 

 first, by examining the specific gravity at different temperatures, by means of the 

 weighing bottle ; and, second, by observing the apparent changes of volume in a 

 thermometric-like vessel. From these experiments, he inferred, that sea- water 

 does not expand when cooled from 40° to its proper freezing point, as fresh- water 

 does. In spite of these experiments of Marcet, the opinion derived from Blag- 

 den continued to prevail. 



The Annalen der Physick und Chemie of Poggendorf, for the year 1828, 

 contains a dissertation by Mr G. A. Erman junior, entitled " Observations on the 

 Expansion of Salt-water by a temperatm-e between -i- 8° and — 3° R. = 50° and 

 25^° Fahr. From his experiments he came to the same conclusion which Marcet 

 had adopted. 



Having taught the doctrine of Blagden for half a century, I felt unwilling to 

 relinquish it upon the authority of the experiments of Marcet or Erman, to 

 the latter of which my attention had been called last summer by the allusion 

 made to them by Mr Lyell, in the last edition of his Geology, and therefore de- 

 termined to investigate the fact by experiments upon ai-tificial solutions, and the 

 natural one presented in sea- water. 



I shall preface the detail of my own experiments by a more particular ac- 

 count of those of Blagden and Erman than the short allusion already made to 

 them. 



I beg leave to quote the words of Sir Charles Blagden, Philosophical Trans- 

 actions, vol. Ixxviii. for year 1788, p. 311. " I shall conclude this paper with the 

 account of an experiment to determine the effect of salt upon the expansion of 

 water by cold. Piu-e water begins to shew this expansion about the temperatiu-e 

 of 40°, that is 8° above its freezing point. I put a solution of common salt, in the 

 proportions of 4.8 pai-ts of water to one of the salt, and consequently whose freez- 

 ing point was 8f °, into an apparatus I had used for other experiments of the same 

 kind, and found that the solution continued to contract till it was cooled to 17°, 

 but had sensibly expanded by the time it was cooled to 15°. Suppose the expan- 

 sion to have begun at 16^°, it would be just 8° above its new freezing point. 

 Hence we have reason to conclude, as far as one experiment goes, that the com- 

 bination of a salt with water has no other effect upon its quality of expanding by 



