1818.] Mr. Holt's Meteorological Journal. 123 



Societies wishing to make an exchange of Transactions may 

 address themselves as follows : — " To Professor Gotthelf 

 Fischer, at Moscow." 



Or if they will send a copy of their Transactions, they may 

 rely with perfect confidence that Professor Fischer will return in 

 their place the works of the Imperial Society of Naturalists. 



Article XI. 



Meteorological Journal for the City of Cork. By T. Holt, Esq. 



(To Dr. Thomson.) 



SIR, Cork, May. 21, 1S18. 



I have been prevented by severe illness from transmitting 

 the annexed meteorological scale (Plate LXXXII) before this 

 time, but trust it is not now too late for insertion in the Annals 

 of Philosophy for July. 1 shall be enabled to complete the 

 continuation of the scale up to June 30, so as to transmit it by 

 July 10 ; and 1 hope in future to have every quarterly scale ready 

 in as short a time after its completion. 



Permit me to inquire, Sir, if it has been before noticed that a 

 dilute solution of indigo in sulphuric acid, when treated with 

 any deoxydizing subject, as bright iron or zinc filings in a closed 

 vessel, loses its blue colour and remains colourless (or rather of 

 a very pale green colour) so long as the vessel is kept closed l 

 The admission of air restores the blue colour, which is again 

 made to disappear by closing the vessel. As on the addition 

 of the iron filings hydrogen gas is evolved, the vessel will 

 require to be occasionally unclosed to let it escape, or it Avill 

 endanger the bursting of the vessel. 



With much respect, I am, Sir, 



Your very obedient, humble servant, 



Thomas Holt. 



The fact related by our correspondent is, we believe, new ; 

 and may, probably, be of considerable practical utility. It is 

 well known that indigo becomes a fast colour only when it is 

 applied in a deoxygenated state ; and this is usually effected by 

 digesting the indigo in warm or cold water with sulphate of iron 

 and lime. The blues, however, thus produced, are not equal in 

 vividness of tint to the Saxon blue, which is made by digesting 

 the indigo in sulphuric acid, by which it is dissolved without 

 undergoing any change of colour and consequent debxygena- 

 tion. On this account the Saxon blue, however beautiful, is a 

 colour more or less fugitive, and, therefore, inapplicable to 

 many purposes. It is possible that by judiciously following up 

 the hint thrown out by our correspondent, a method may be 



