Db. Arnott on the Measurements of Heights. 



Experiment. Calculation. 



Chromic acid, 51-840 52-52 



Potash, 23-900 24-24 



Lime, 14-950 14-14 



Water, 9-600 9-10 



100-290 100- 



This corresponds nearly with the formula KO CrOj CaO CrOs + 2 HO. 

 The water being slightly in excess. It, therefore, is a parallel compound 

 to the magnesian salt described by Dr. Thomson. 



Orange Potash Chromaie of Lime. — The mode of formation of this salt 

 has been already described. It is soluble in water. The mean of three 

 analyses yielded the following result : — 



Chromic acid, 52070 



Lime, 23-990 



Potash, 17-550 



Water, 6230 



99-840 



This approaches nearly to the formula 3 KO 7 CaO 7 CrOs 5 HO. 

 When this salt is ignited it does not fuse, and when cool its colour is 

 yellow. It does not again altogether dissolve in water, and thence it 

 appears to have undergone decomposition. 



1 1 . — Notice regarding the Measurements of Heights, by Means of the Boiling 



Pohrt of Water. By Gr. A. Walkek Arnott, LL,D., Regius Professor 



of Botany. 



At the last meeting of this Society, a paragraph was read from one of 

 Dr. Thomas Thomson's (jun.) letters from the mountains of Northern India, 

 to the following effect : — 



" Time, 6J p.m., temperature of air 80°, water boiled at 203-6. By the 

 way, 545 feet for a degi-ee is too little, at least for considerable heights ; 

 nor will a constant multiplier answer at all. Professor Forbes has some 

 number near 560 [it is only 549|^,] for a constant multiplier, but that was 

 found gave quite erroneous results at great heights. There remains still 

 something to be done in the matter. What the thermometer really gives, 

 in fact, is the pressure of the atmosphere, and its indications vary with 

 that." 



While making some unpremeditated remarks to the meeting on Dr. 

 Thomson's interesting letters, I mentioned that no one doubted that the 

 thermometer merely indicated the pressure of the air ; but that its varia- 

 tions were not proportional to the difference of pressure, but to the 



