Mr Parkes, in his Chemical Essays, published in 1815, has mentioned some facts upon this subject, which it will be proper to state. In the early part of the year 1787, Professor Copland of Aberdeen accompanied the present, Duke of Gordon to Geneva, and he was shown the discolouring property of oxy muriatic acid by M. de Sausiture. .Mr Copland was much struck with the importance of the experiment, and on his return to Aberdeen in July 1787, men tioned the circumstance, and repeated the experi ment before some eminent bleachers in his own neighbourhood. These gentlemen were Messrs Milnes of the house of Gordon, Barron, and Com pany, Aberdeen. They immediately began the al). plication of the process to the bleaching of linen on • a great scale ; and Mr Parkes assures us that they were the first persons who applied the new process to practical bleaching in Great Britain.
But this statement, though it may appear plausi ble at first sight, is quite incorrect. The writer of this article took the liberty of applying to Mr Watt himself for information on the subject. Mr Watt has preserved copies of all his letters since the year 1782, taken by means of his copying machine. He allowed the writer of this article to peruse such of them as bore any reference to this subject. Now, two letters were found, which entirely set the matter at rest. The first of these is to his father-in-law, Mr Macgre gor, dated Birmingham, March i 9, 1787. In this letter he gives a particular detail of the new bleach ing process, states its advantages, and says that he had sent Mr Macgregor a quantity of the whitening liquor. The second letter is to Berthollet, and is dated Birmingham, May 9, 1787. The following is a part of that letter, which we have transcribed ver batim : " Je ne sail pas si j'ai encore fait la liqueur acid si fort que vous ayes fait, mais je you. donnerois les moyens de juger. Je trouve que 4 onces de mon acide mele avec la quantite necessaire d'alkali de pearl-ash peat blanchir un gros de toile brune, tale comme j'ai vu chez vous. II est vrai qu'il ne la fait tout a fait blanc ; mais it le fait aussi blanc, que je puis le faire, meme en ajoutant une second dose d'acide. Je hotline la toile par avance dans une so lution d'alkali faible ; et a mi blanc, je Is bouille une second foie. Je trouve que le seven est meilleur que l'alkali pur pour la second bouillon. J'ai chi toute a fait le coton, mais je ne suis encore par venu a blanchir parfaitement la Wile de lin." The reader will observe, that the date of both of these letters is some months before Mr Copland's return from the Continent. M. Berthollet, had published
his process in 1785, and as Watt had brought it to England in the end of 1786, and had put it in prac tice, and introduced it into Mr Macgregor's bleach field, near Glasgow, in the month of March 1787, it is clear that Saussure has no claim to the original discovery, nor Mr Copland to the first introduction of the new process into Great Britain.
Dr Henry quotes a letter of Mr Watt, dated 1788, February 23, in which he says, " I have for more than a twelvemonth been in possession and practice of a method of preparing a liquor from common salt, which possesses bleaching qualities in an eminent de gree; but not being the inventor, I have not at tempted to get a patent or exclusive privilege for it." (Annals of Philosophy, VI. 428.) This letter alone is sufficient to show, that Mr Watt's experiments were of an earlier date than those of Messrs Milnes. He says, farther, that " at that very time 1500 yards 'of linen were bleaching by the new process, under his directions." This great experiment was con• ducted in the bleachfield of his father-in-law, Mr Macgregor, near Glasgow ; where, as he wrote to M. Berthollet, soon after, 500 pieces were bleached by the new method, and Mr Macgregor was so sa tisfied of the importance of the new process, that he resolved to continue it. Mr Watt made several im provements in the method of M. Berthollet. Instead of employing rouriatic acid and manganese, as had been done by Scheele and Berthollet, he had recourse to the cheaper mode of a mixture of coin. mon salt, black oxide of manganese, and sulphuric acid. He made use of wooden vessels to hold the water, which was tq be impregnated with the oxy muriatic gas, coating them within with a mixture of wax and pitch, which rendered the air light, and prevented the gas from acting on the wood. Mr Watt likewise contrived a test to indicate the strength of the water impregnated with oxymuriatie acid, as far as its bleaching effects were concerned. He took a determinate quantity of the infusion of cochineal, and ascertained how much of the bleach ing liquor was necessary to destroy the colour. The strength of the bleaching liquor was obviously in versely as the quantity necessary to destroy the co lour. But M. Welter hit upon another method about the same time, which has been considered as preferable, and has in consequence come into gene ral use. He employed a solution of indigo in sul phuric acid instead of the infusion of cochineal. In other respects the two methods were the same.