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Meeting Dr。 Wollaston one morning in the shop of a bookseller； I proposed this question： If two volumes of hydrogen and one of oxygen are mixed together in a vessel； and if by mechanical pressure they can be so condensed as to become of the same specific gravity as water； will the gases under these circumstances unite and form water？ 〃What do you think they will do？〃 said Dr。 W。 I replied； that I should rather expect they would unite。 〃I see no reason to suppose it；〃 said he。 I then inquired whether he thought the experiment worth making。 He answered； that he did not； for that he should think it would certainly not succeed。
A few days after； I proposed the same question to Sir Humphry Davy。 He at once said； 〃they will become water； of course；〃 and on my inquiring whether he thought the experiment worth making； he observed that it was a good experiment； but one which it was hardly necessary to make； as it must succeed。
These were off…hand answers； which it might perhaps be hardly fair to have recorded； had they been of persons of less eminent talent： and it adds to the curiosity of the circumstance to mention； that I believe Dr。 Wollaston's reason for supposing no union would take place； arose from the nature of the electrical relations of the two gases remaining unchanged； an objection which did not weigh with the philosopher whose discoveries had given birth to it。
＇The result of the experiment appeared； and still appears to me； to be of the highest importance； and I will shortly state the views with which it was connected。 The next great discovery in chemistry to definite proportions； will be to find means of forming all the simple unions of one atom with one； with two； or with more of say other substance： and it occurred to me that the gaseous bodies presented the fairest chance of success； and that if wishing； for instance； to unite four atoms of one substance with one of another； we could； by mechanical means； reduce the mixed gases to the same specific gravity as the substance would possess which resulted from their union； then either that such union would actually take place； or the particles of the two substances would be most favourably situated for the action of caloric； electricity； or other causes； to produce the combination。 It would indeed seem to follow； that if combination should take place under such circumstances； then the most probable proportion in which the atoms would unite； should be that which furnished a fluid of the least specific gravity： but until the experiments are made； it is by no means certain that other combinations might not be produced。＇
The singular minuteness of the particles of bodies submitted by Dr。 Wollaston to chemical analysis； has excited the admiration of all those who have had the good fortune to witness his experiments； and the methods he employed deserve to be much more widely known。
It appears to me that a great mistake exists on the subject。 It has been adduced as one of those facts which prove the extraordinary acuteness of the bodily senses of the individual； a circumstance which； if it were true； would add but little to his philosophical character； I am； however； inclined to view it in a far different light； and to see in it one of the natural results of the admirable precision of his knowledge。
During the many opportunities I have enjoyed of seeing his minute experiments； I remember but one instance in which I noticed any remarkable difference in the acuteness of his bodily faculties； either of his hearing； his sight； or of his sense of smell； from those of other persons who possessed them in a good degree。 ＇This was at Mr。 South's observatory； and the object was； the dots on the declination circle of his equatorial； but； in this instance； Dr。 Wollaston did not attempt to TEACH ME HOW TO SEE THEM。＇
He never showed me an almost microscopic wire； which was visible to his； and invisible to my own eye： even in the beautiful experiments he made relative to sounds inaudible to certain ears； he never produced a tone which was unheard by mine； although sensible to his ear； and I believe this will be found to have been the case by most of those whose minds had been much accustomed to experimental inquiries； and who possessed their faculties unimpaired by illness or by age。
It was a much more valuable property on which the success of such inquiries depended。 It arose from the perfect attention which he could command； and the minute precision with which he examined every object。 A striking illustration of the fact that an object is frequently not seen； FROM NOT KNOWING HOW TO SEE IT； rather than from any defect in the organ of vision； occurred to me some years since； when on a visit at Slough。 Conversing with Mr。 Herschel on the dark lines seen in the solar spectrum by Fraunhofer； he inquired whether I had seen them； and on my replying in the negative； and expressing a great desire to see them； he mentioned the extreme difficulty he had had； even with Fraunhofer's description in his hand and the long time which it had cost him in detecting them。 My friend then added； 〃I will prepare the apparatus； and put you in such a position that they shall be visible； and yet you shall look for them and not find them： after which； while you remain in the same position； I will instruct you how to see them； and you shall see them； and not merely wonder you did not see them before； but you shall find it impossible to look at the spectrum without seeing them。〃
On looking as I was directed； notwithstanding the previous warning； I did not see them； and after some time I inquired how they might be seen； when the prediction of Mr。 Herschel was completely fulfilled。
It was this attention to minute phenomena which Dr。 Wollaston applied with such powerful effect to chemistry。 In the ordinary cases of precipitation the cloudiness is visible in a single drop as well as in a gallon of a solution； and in those cases where the cloudiness is so slight； as to require a mass of fluid to render it visible； previous evaporation； quickly performed on slips of window glass； rendered the solution more concentrated。
The true value of this minute chemistry arises from its cheapness and the extreme rapidity with which it can be accomplished： it may； in hands like those of Wollaston； be used for discovery； but not for measure。 I have thought it more necessary to place this subject on what I consider its true grounds； for two reasons。 In the first place； I feel that injustice has been done to a distinguished philosopher in attributing to some of his bodily senses that excellence which I think is proved to have depended on the admirable training of his intellectual faculties。 And； in the next place； if I have established the fact； whilst it affords us better means of judging of such observations as lay claim to an accuracy 〃MORE THAN HUMAN；〃 it also opens； to the patient inquirer into truth； a path by which he may acquire powers that he would otherwise have thought were only the gift of nature to a favoured few。
APPENDIX； No。 1。
In presenting to my readers the account of the meeting of men of science at Berlin； in the autumn of 1828； I am happy to be able to s