458 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



absorbing the dephlogisticated air ; and when I first read the account, I thought 

 this the most probable cause of the difference in the result of our experiments ; 

 but I am now inclined to think that the case was otherwise. From the manner 

 in which M. Van Marum expresses himself, it seems that the only circumstance, 

 from which they concluded that the alkali was not saturated, was the imperfect 

 marks of deflagration that the paper dipped into it exhibited in burning; which, 

 as we have seen, might proceed as well from some of the mercury having been 

 dissolved as from the alkali not being saturated. I am much inclined to think 

 therefore, that, so far from the soap-lees not having been saturated, the quantity 

 of acid produced was in reality much more than sufficient for this purpose, and 

 had dissolved a good deal of the mercury; for the quantity of air absorbed favours 

 this opinion, and the phenomena agree well with Mr. Gilpin's first experiment, 

 in which this was certainly the case; whereas, if the diminution had proceeded 

 chiefly from the dephlogisticated air being absorbed by the iron, the tube towards 

 the end of the experiment would have been filled chiefly with phlogisticated air, 

 which would have made the diminution proceed much slower than before; but 

 we are told that it went on as fast as ever. It is most likely therefore, that the 

 apparent disagreement between their experiment and mine, proceeded only from 

 their having continued the process too long, and from their not having properly 

 examined the produce. 



M. Van Marum then proceeds to say : " Surpris de cette difference de resultat, 

 j'envoyai une description exacte de nos experiences a M. Cavendish, le priant en 

 meme tems de m'instruire s'il pourroit trouver la cause de cette difference; et 

 comme la seule difference essentielle, par laquelle notre experience differoit de 

 celle de M. Cavendish, consistoit en ce que nous avons employe de I'air pur pro- 

 duit du precipite rouge ou du minium, au lieu de I'air pur produit de la poudre 

 noire formee par I'agitation du mercure avec le plomb, dont M. Cavendish ne 

 donne pas la maniere de le produire,* je le priai de me communiquer de quelle 

 maniere il etoit venu a cet air, parceque je desirois de repeter I'experience avec ce 

 meme air: mais comme il ne m'a fourni aucune elucidation sur la cause vraisem- 

 blable de la difference du resultat de nos experiences, et qu'il ne lui a pas plu de 

 me communiquer sa maniere de produire I'air pur qu'il avoit employe pour ses 

 experiences, m'ecrivant, qu'il s'etoit propose d'en parler dans un ecrit public, la 

 longueur ennuyante de ces experiences nous a fait prendre la resolution de dif- 

 ferer leur continuation, pour obtenir uneparfaite saturation de la lessive, jusqu'4 



* The using the iron wire formed a material difference in our manner of conducting the experi- 

 ment, and one which may perhaps have had great influeace on the result ; but I do not see how the 

 using some other kind of dephlogisticated air, instead of that prepared from Dr. Priestley's black 

 powder, can in the least degree form an essential difference, as in the same paragraph in which I 

 mention my having used this kind of air in my first experiment, I say, that in my second experi- 

 ment I used air prepared from turbith mineral. — Orig. 



