Mar. 4. 1854.] 



NOTES AND QUERIES. 



207 



sition, no change could take place, the affinities of acetic 

 acid being insufficient to displace hydrobromic acid. 



A great many experiments which I tried last au- 

 tumn, for the express purpose of clearing up this point, 

 have convinced me that, cceteris paribus, the addition of 

 free iodine to the iodizing solution, tends to diminish 

 the sensitiveness of the subsequently formed iodide of 

 silver. On paper, this diminution of sensitiveness is 

 attended with some advantages, so that at present I 

 hardly know whether to introduce the free iodine or 

 not ; but in collodion, as far as my experience goes, I 

 see no reason for retaining it ; on the contrary, every- 

 thing seems to be in favour of its removal. 



I can hardly imagine that the increased sensitiveness 

 mentioned by Mr. Hennah is really due to the free 

 iodine which he introduces. Such a result being so 

 contrary to all my experience, I would venture to 

 suggest that there must be some other cause for its 

 beneficial action ; for instance, commercial iodide of 

 potassium is generally alkaline, owing to impurities 

 present ; the tincture of iodine in this case would 

 render the collodion neutral, and unless a very large 

 excess of iodine were introduced, its good effects would 

 be very apparent. This, however, involving the em- 

 ployment of impure chemicals, is a very improbable ex- 

 planation of a phenomenon observed by so excellent an 

 operator as Mr. Hennah : there is most likely some 

 local cause which would be overlooked unless expressly 

 searched for. 



With regard to the point, whether the free iodine is 

 the sole cause of the deterioration of old collodion, I 

 should say decidedly not, at least in a theoretical view ; 

 the liberation of free iodine necessitates some other 

 changes in the collodion, and the result must be in- 

 fluenced by these in one way or another, but prac- 

 tically I have as yet found nothing to warrant the 

 supposition that they perceptibly interfere with the 

 sensitiveness of the film. 



In the above I have endeavoured as much as pos- 

 sible to avoid technicalities, in order to make it intel- 

 ligible to amateurs ; but if there be any part which 

 may be considered obscure, on its being pointed out to 

 me, I will endeavour to solve the difficulty. 



William Crookes, 



Hammersmith. 



Photographic Queries. — 1. Would you, Sir, or Dr. 

 Diamond (Dr. Mansell is too far off), be kind 

 enough to inform your readers whether Dr. Mansell's 

 process, recommended in No. 225., is equally appli- 

 cable to inland as to sea-side operations ; or must we, 

 in the one case, follow Dr. Diamond, and in the other 

 Dr. Mansell, and thus be compelled to prepare two 

 sets of papers ? 



2. Dr. Mansell recommends, as a test for the 

 iodized paper, a strong solution of bichloride of mer- 

 cury ; may we ask how strong ? 



3. Mr. Sisson's developing fluid has undergone so 

 many changes, and has been so much written about, 

 that we are at a loss to discover or to determine 

 whether it has been at length settled, in the mind of 

 the inventor, that it will do equally well for negatives 

 as for positives. Four Photographic Readers. 



[1. Both papers are equally available for both pur- 



poses. In actual practice we have not ourselves ex- 

 perienced any difference in their results. 



2. It is quite immaterial. A drachm of bichloride 

 dissolved in one ounce of spirits of wine will cause a 

 cloudiness and a precipitate, if a very few drops are 

 added to the tested water. 



3. In general the salts of iron are more adapted for 

 positives, and weak pyrogallic acid solutions for nega- 

 tives ; say one and a half grain of pyrogallic acid, 

 twenty minims of glacial acetic acid, and an ounce of 

 distilled water.] 



3SUpItej> to fflinav <®utvtt&. 



London Fortifications (Vol. ix., p. 174.). — In 

 last week's Number is an inquiry as to " London 

 Fortifications" in the time of the Commonwealth. 



There is a Map by Vertue, dated 1738, in a 

 folio History of London; there is one a trifle 

 smaller, copied from the above ; also one with 

 page of description, Gentleman's Magazine, June, 

 1749. I subscribed to a set of twenty etchings, 

 published last year by Mr. P. Thompson of the 

 New Road; they are very curious, being fac- 

 similes of a set of drawings done by a Capt. John 

 Eyre of Oliver Cromwell's own regiment, dated 

 1643. The drawings are now I believe in the 

 possession of the City of London. 



A Constant Reader. 



[The drawings referred to by our correspondent are, 

 we hear, by competent judges regarded as not genuine. 

 Such also, we are told, is the opinion given of many 

 drawings ascribed to Hollar and Captain John Eyre, 

 which have been purchased by a gentleman of our 

 acquaintance, and submitted by him to persons most 

 conversant with such drawings. Query, Are the 

 drawings purporting to be by Captain John Eyre, 

 drawings of the period at which they are dated?] 



Burke's Domestic Correspondence (Vol. ix., 

 p. 9.). — In reference to a Query in " N. & Q." 

 relative to unpublished documents respecting Ed- 

 mund Burke, I beg to inform your correspondent 

 N. O. that I have no doubt but that some new 

 light might be thrown on the subject by an appli- 

 cation to Mr. George Shackleton, Ballitore, a de- 

 scendant of Abraham Shackleton, Burke's old 

 schoolmaster, who I believe has a quantity of 

 letters written to his old master Abraham, and 

 also to his son Richard, who had Burke for a 

 schoolfellow, and continued the friendship after- 

 wards, both by writing and personally. When 

 Richard attended yearly meetings in London, he 

 was always a guest at Beaconsfield. Burke was 

 so much attached to Richard, that on one of 

 these visits he caused Shackleton's portrait to be 

 painted and presented it to him, and it is now in 

 the possession of the above family. I have no 

 doubt but that an application to the above gentle- 

 man would produce some testimony. F. H. 



