Mr. Daniell on the Barometer. 87 



been describing, it is necessary that both glass and mercury 

 should be in the driest and cleanest possible state ; that is to say, 

 exactly in the state in which they exist in a well made baro- 

 meter. 



That another attempt has been made at explanation I can 

 scarcely credit ; namely, that some facetious person had played 

 Mr. Faraday a trick. The particulars of the case disprove the 

 possibility of such a circumstance, unless upon the supposition 

 that such person foresaw the present discussion. The character, 

 however, of Mr. Faraday for precision, renders it unnecessary to 

 say any more, than that he informs me there is not the slightest 

 ground for such a suspicion. 



I was no sooner convinced that the most carefully constructed 

 barometers were liable to a slow and gradual deterioration in the 

 manner which I have indicated, than I endeavoured to find a 

 remedy to the evil ; without which, it is clear, that some of the 

 most interesting problems of meteorology must be for ever left 

 in a state of vagueness and uncertainty. For a long time I de- 

 spaired of success ; but when at length I discovered an effectual 

 method of preserving the Torricellian vacuum, I flattered myself 

 that the Royal Society would so far have taken an interest in the 

 subject as to have ordered it to be submitted to a trial, which 

 could not have been so satisfactorily made under any other super- 

 intendence. 



I soon perceived that the only possibility of effecting the ob- 

 ject which I had in view, consisted in discovering some method 

 of making the mercury wet (if I may be allowed the term) 

 the tube in which it is contained. I was fearful, at first, that all 

 the substances to which its attraction is sufficiently strong for 

 this purpose, would be so much acted upon as to become disinte- 

 grated or dissolved. I, however, fortunately recollected that, in 

 some experiments in which I was formerly engaged, platinum, 

 immersed in boiling mercury, became completely coated by it, 

 and afterwards retained its coating for a long time. I repeated 

 the experiment with some platinum foil, and found it to succeed 

 perfectly. The mercury adhered strongly to the foil, and the 

 latter, after a long immersion, was found to have lost none of its 



