1856] WRITINGS OP JOSEPH HENRY. 423 



vacuum, but nearly pure nitrogen: the oxygen had been 

 absorbed in passing through the oil, producing rancidity 

 and other changes in that liquid. 



It was evident from this experiment that oil was not im- 

 pervious to air. Another attempt to remedy this defect of 

 the instrument was made by using a thin film of gutta- 

 percha, to be left after the evaporation of the naptha in 

 which it had been dissolved. 



An objection however to the use of water as the liquid for 

 the barometer is the vapor which it always gives off, and of 

 which the tension cannot readily be determined. In a glass 

 vessel in which a cup of water is enclosed. Professor Espy 

 informs me that he has found the dew-point always less than 

 that which would be due to the temperature. 



Desiring to fit up a barometer on a large scale as one of the 

 objects of interest and use in the Smithsonian Institution, 

 I consulted my friend Professor G. C. Schaeffer of the Patent 

 Office, as to the best liquid to be employed. He advised the 

 use of sulphuric acid, but I did not immediately adopt his 

 advice on account of the apparently dangerous character of 

 this substance. Happening however some time afterwards 

 to be speaking on the subject of barometers with Mr. James 

 Green, the instrument-maker, in the presence of Professor 

 Ellet, of New York, the latter asked why I did not have a 

 large one constructed with sulphuric acid. The suggestion 

 having thus again been independently made, and Mr. Green 

 expressing his willingness to undertake the work, I gave the 

 order for the construction of the instrument, and requested 

 Professor Ellet to give any suggestions as to the details 

 which might be required. 



The advantages of this liquid are: 1. That it gives off no 

 appreciable vapor at any atmospheric temperature ; and 2. 

 That it does not absorb or transmit air. The objections to 

 its use are: 1. The liability to accident from the corrosive 

 nature of the liquid, either in the filling of the tube or in its 

 subsequent breakage; and 2. Its affinity for moisture, which 

 tends to produce a change in specific gravity. The filling 

 however is a simple process and attended with but little if 



