Mr. J. H. Alexander on the Tension of Vapour of Wate7\ 7 



And yet, after all, it does not include the whole of our ex- 

 perimental data. Those of John Henry Ziegler, of Winter- 

 thiir in the Canton of Zurich (whose name I give at length 

 because he may be regarded as having led the way in this 

 research), of Achard, of Schmidt, of Magnus, and of some 

 others whose names escape me at the moment or have failed 

 to come to my knowledge at any time, — either were not ac- 

 cessible to me at all, or only in statements at second or third 

 hand, whose accuracy I had not the means of verifying. 



Nor does it contain quite all the results of those observers 

 to whose experiments I had access. To have given every 

 one of each, would have rendered the table, in respect to the 

 present aim, quite enormous. For instance, the several series 

 of M. Regnault, the latest experimentalist, exhibit such a 

 luxury of repetition, at temperatures varying very slightly — 

 sometimes only by small fractions of a degree — as almost to 

 outnumber, in themselves alone, all the quotations 1 have 

 made from others. I exercised, therefore, the discretion of 

 using only those instances which, at reasonable degrees apart, 

 rested upon a number of concurrent observations. In general, 

 such concurrences of the same mean temperature give the 

 mean of three observations of pressure ; the result for 32*^ F, 

 is the mean oi forty seven recorded pressures. For the ex- 

 periments earlier than Mr. Southern's, I have usually taken 

 only those whose recorded temperatures were already other- 

 wise in the table. I shall have occasion, however, to speak of 

 each one more particularly, presently. 



The difference of apparatus and manipulations necessary 

 for experimenting upon the elasticity of vapour above and 

 below the boiling temperature, has led several of the observers, 

 for convenience or other motives, to confine themselves to one 

 or the other side of this limit; and would render proper, in 

 any discussion of the value and reliability of such observa- 

 tions, an arrangement of them in two distinct tables. But as 

 I have no such aim at present, and as the exemplification of 

 the formula belongs equally to the lowest and highest tempera- 

 tures, there is no reason for breaking the continuity of the com- 

 parison or for presenting the results in more than one table. 



The order of the different columns is chronological, ac- 

 cording to the dates of execution ; or, when that was not 

 known, of publication of the experiments. As in fact each 

 successive observer had, or might have had, the benefit of the 

 experience and precautions of his predecessor, it may be pre- 

 sumed that this order represents too, in a measure, the 

 respective reliabilities of the results. Only the two French 

 series, while they are evidently unrivalled in the extent to 

 which, in opposite directions, they have been carried, are 

 similarly distinguished by the refined elaboration which cha- 

 racterizes every part of the research. 



