VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 45Q 



ce que M. Cavendish ait public sa maniere de produire I'air pur, dont il s'est servi; 

 nous contentant pour le present d'avoir vu, que I'union du principe d'air pur 

 et de la mofette produit lel'acide nitreux, suivantla decouverte de M. Cavendish." 

 As I should be sorry to be thought to have refused any necessary information 

 to a gentleman who was desirous to repeat one of my experiments, and who by 

 his situation was able to do it with less trouble than any one else, I hope the 

 society will indulge me in adding a copy of my answer, that they may judge 

 whether this is in any degree a fair representation of it. 



" To M. Fan Marum. 

 " Sir, — I received the honour of your letter, in which you inform me of your 

 ill success in trying my experiment on the conversion of air into nitrous acid by 

 the electric spark. It is very difficult to guess why an experiment does not suc- 

 ceed, unless one is present and sees it tried; but if you intend to repeat the 

 experiment, your best way will be to try it with the same kind of apparatus that 

 I described in that paper. If you do so, and observe the precautions there 

 mentioned, I flatter myself you will find it succeed. The apparatus you used seems 

 objectionable, on account of the danger of the iron being corroded by absorbing 

 the dephlogisticated air. As to the dephlogisticated air procured from the black 

 powder formed by agitating mercury mixed with lead, as it was foreign to the 

 subject of the paper, and as 1 proposed to speak of it in another place, I did 

 not describe my method of procuring it. As far as I can perceive, the success 

 depends entirely on carefully avoiding every thing by which the powder can absorb 

 fixed air, or become mixed with particles of an animal or vegetable nature, or 

 any other inflammable matter: for which reason care should be taken not to 

 change the air in the bottle in which the mercury is shaken, by breathing 

 into it as Dr. Priestley did, or even by blowing into it with a bellows, 

 as thereby some of the dust from the bellows may be blown into it. 

 The method which I used to change the air was, to suck it out by means 

 of an air-pump, through a tube which entered into the bottle, and did 

 not fill up the mouth so close but what air could enter in from without, to supply 

 the place of that drawn out through the tube. — I am, &c." 



With regard to the main experiment, it was not in my power to give him 

 further information than I did; as I pointed out the only circumstance to which, 

 at that time, I could attribute the difference in our results. And with regard to 

 the manner of preparing the dephlogisticated air from the black powder, I have 

 mentioned all the particulars in which my manner of proceeding differed from 

 Dr. Priestley's, and have also explained on what I imagine the success entirely 

 depends; so that I believe no one, at all conversant in this kind of experiments, 

 will think that I did not communicate to him my method of procuring that air. 



XFIII. Experiments on the Effect of various Substances in Lowering the Point 



3 N 2 



