Improvements on a Dew-point Hygrometer. 293 



The formula of this salt is therefore — 



2(As05 6NaO SnO^) 50HO. 



Stannate of soda is used as a mordant in print and dye works. 

 I have made numerous experiments on a large scale to decide 

 whether in cahco printing or dyeing, the hydrated peroxide of 

 tin alone would be preferable to an arseniate of peroxide of tin 

 as a mordant. The result of comparative trials leads me to give 

 the preference to hydrated pei'oxide of tin alone, the various 

 shades being more brilliant and less liable to unevenness than 

 when arsenic acid is present. It would be desirable to substi- 

 tute in commerce a purer stannate of soda for that at present 

 sold, as well as for that containing arseniate of soda. 



These experiments were made in the laboratory of Harrison 

 Blair, Esq., near Manchester, who kindly afforded me every op- 

 portunity for carrying out the same. They have led at least to 

 this practical fact, that the danger arising from the employment 

 of arseniates in stannates of soda may be obviated by the use of 

 a pui-e stannate of soda alone. 



XXXVI. Improvements on a Dew-point Hygrometer lately de- 

 scribed by the Author^. By Professor CoNNELLf. 



A FEW simple alterations have greatly facilitated the em- 

 ployment of this instrument, and at the same time made 

 it considerably less liable to injury. It consists, as will be recol- 

 lected, of a little bottle of thin brass, into which the bulb of a 

 thermometer is introduced so as to fit air-tight, and the neck of 

 which is attached laterally to a small exhausting syringe. iEther 

 having been previously introduced into the bottle, the tempera- 

 ture is by a gradual process of exhaustion reduced, until moisture 

 begins to be condensed on the polished surface of the bottle, 

 when the temperature of deposition is indicated by the thermo- 

 meter, A little collar of ivory is introduced between the bottle 

 and the syringe to prevent the heat produced by the friction of 

 the piston and cylinder from being communicated to the bottle. 

 The valves of the syringe are made of gold-beater's leaf, as the 

 vapour of the aether was found to act on valves of oiled silk. 



As washers of leather required to be introduced on both sides 

 of the ivory collar to make the connexions air-tight, considerable 

 difficulty was experienced, from the varying thickness of these 



* Edinburgh Philosophical Transactions for 1854, and Philosophical 

 Magazine, vol. viii. p. 8). 



t Coinmiuiieated by the Author, having been read to the Physical 

 Section of the British Association at Glasgow, September 17, 1855. 



