PHILOSOPHICAL SOCIETY OP WASHINGTON. 333 



difference of density between the original liquor and that from 

 the top or bottom of the column, after the lapse of hours, days, 

 weeks, or months. The fluid at the bottom of the tube it must 

 be remembered was for five months exposed to the pressure of a 

 column of fluid at least one hundred feet high." * 



Sulphuric-acid Barometer.— In 1856, Henry had constructed 

 for the Smithsonian Institution, at the suggestion of Professor 

 G. C. Schaeffer, a large sulphuric acid barometer, whose column 

 being more than seven times the height of the mercurial column 

 (about 18^ feet) gave correspondingly enlarged and sensitive indi- 

 cations. Water barometers with cisterns protected by oil, (as that 

 constructed by Daniell for the Royal Society,) have always proved 

 instable. With reference to sulphuric acid, " The advantages of 

 this liquid are : 1st that it gives off no appreciable vapor at any 

 atmospheric temperature; and 2nd that it does not absorb or 

 transmit air. The objections to its use are : 1st the liability to 

 accident from the corrosive nature of the liquid, either in the 

 filling of the tube or in its subsequent breakage ; and 2nd its 

 affinity for moisture, which tends to produce a change in specific 

 gravity. " The latter defect was obviated by a drying apparatus 

 consisting of a tubulated bottle containing chloride of calcium, 

 and connected by a tube with the glass bottle forming the reser- 

 voir, which excluded all moisture from the transmitted air. " The 

 glass tube [of the barometer] is two hundred and forty inches 

 long, and three-fourths of an inch in diameter ; and is enclosed 

 in a cylindrical brass case of the same length, and two and a 

 half inches in diameter. The glass tube is secured in the axis 

 of the brass case by a number of cork collars, placed at inter- 

 vals."! This barometer continued in successful and satisfactory 

 use for many years ; and had its readings constantly recorded. 



Of several of Henry's courses of experiments, no details have 

 been published ; and his original notes appear to have perished. 

 In 1861, he made a number of experiments on the effects of 

 burning gunpowder in a vacuum, as well as in different gases. 



"A series of researches was also commenced, to determine 

 more accurately than has yet been done, the expansion produced 

 m a bar of iron at the moment of magnetization of the metal by 

 means of a galvanic current. The opportunity was taken with 

 the consent of Professor Bache, of making these experiments 

 with the delicate instruments which had previously been employed 

 in determining the varying length, under different temperatures 

 ot the measuring apparatus of the base lines of the United States 

 Coast Survey. "| This wonderfully microscopic measuring appa- 



* Proceed. Am. Ai<r.oc. Providence, Au^. 1855, pp. 142, 143. 

 t Proceed. Am. Assoc. Albany, Avis. 1856, pp. 135-138. 

 X Smithsonian Report for 1861, p. 38. 



107 



