1819.] Scientific Intelligence. 383 
the important discovery of a crystallized persulphate of iron, I 
think it necessary to state, that in the month of October last 
year, Mr. Rennie, a friend of mine, a surgeon in Glasgow, a 
very ingenious man and fond of chemistry, brought me a few 
crystals, which were obtained, he said, by evaporating green 
vitriol repeatedly in the open air, redissolving it by means of 
sulphuric acid, concentrating the solution, and setting it aside. 
He succeeded only once in obtaining these crystals, and he 
considered them as crystals of persulphate of iron. These erys- 
tals were regular octahedrons (as far as could be determined by 
the eye), they were transparent and colourless, and had very 
much the taste and appearance of alum crystals. The whole 
quantity which I got did not exceed a grain in weight. I dis- 
solved one of these crystals in distilled water, added to the 
solution an excess of caustic potash, heated and then poured the 
colourless liquid off the peroxide of iron which had been preci- 
pitated. On adding sal ammoniac to this liquid, I got a white 
precipitate, which appeared to the eye little less abundant than 
the preceding precipitate of peroxide of iron. From these 
experiments, which were all that the minute quantity of crystals 
in my possession admitted of, I considered the presence of 
alumina in them as ascertained. Hence | was led to suspect the 
presence of alum, to conclude that the salt was a mixture of 
alum and persulphate of iron, and that the crystalline form was 
owing to the alum. This conjecture of mine, which from Mr. 
Cooper’s experiments we see was inaccurate, prevented Mr. Ren- 
nie from making his discovery known at the time. I have still 
one or two of the crystals which he gave me in my possession.* 
I may take this opportunity of stating, that the persulphate of 
iron, which I described in Annals of Philosophy, xn. 462, was 
the same as Mr. Cooper’s in composition, though the shape of 
the crystals was different.——-T. 
IX. Gauze Veils suggested as Preventives of Contagion. 
By Mr. Bartlett. 
(To Dr, Thomson.) | 
SIR, 
Permit me, through the pages of your Annals, to suggest to 
our medical readers, and those employed in Hospitals and other 
infected places, the practicability of using gauze vetls as a 
preventive of contagion. This method was successfully em- 
hing by M. de Saussure and his party when he ascended 
ont Blanc, to preserve their faces from excoriation ; nor was 
their sight at all impaired, as is usually the case with travellers 
in elevated regions. When, therefore, we perceive the efficacy 
of a contrivance so simple in the rarest of mediums, and descend 
to the pesos depths of the earth to which the labour or inge- 
nuity of man has penetrated, and find the same means made use 
* I noticed these crystals before in Annals of Philosophy, xii, 461. 
