Intelligence and Miscellaneous Articles. 467 



by the side of the companion-tube and in the same trough or cistern. 

 Both tubes being supported by cords passing over pulleys, it is easy 

 to bring the level of the mercury in the two tubes to an exact coin- 

 cidence. The pressure on the gas is then the same in each tube. 

 The temperature is also the same, as the tubes hang side by side in 

 the room set apart for gas-analyses, and are equally affected by any 

 thermometric change. It is then only necessary to read off the vo- 

 lumes of the gas in the two tubes to have all the data necessary for 

 calculating the weight of the gas to be measured. This calculation 

 may be effected in two ways, each of which will be found of use. 

 Thus proportion (4) reads in words : — as the observed volume of the 

 air in the companion-tube is to the observed volume of the gas in 

 the measuring-tube, so is the reduced volume of the air in the first 

 (previously determined as above) to the reduced volume of the gas to 

 be measured. This method, of course, applies to the reduction of 

 any gaseous mixture whatever to the normal pressure and tempera- 

 ture. In absolute nitrogen determinations, however, proportion (5) 

 gives the weight of the nitrogen measured at once, since the term 

 w x V is found by multiplying the weight of 1 cubic centim. of nitro- 

 gen at 0° and 760 millims. by the reduced volume of air in the com- 

 panion-tube, and is a constant which can be used as long as the com- 

 panion-tube lasts. In practice, a companion-tube filled with mercury 

 will last with a little care for a very long time. Even when filled 

 with water I have found that excellent results may be obtained, and 

 that the tube M'ill last for some weeks. Williamson and Russell, in 

 their processes for gas-analysis, have employed a companion-tube 

 for bringing a gas to be measured to a constant pressure ; but the 

 application made above is, I believe, wholly new.—Silliman's Ameri- 

 can Journal for May 1870. 



PHYSICAL NOTICES. BY J. MtJLLER. 



I. In the 136th volume of these Annalen I have described a 

 method for determining, by means of a vibration-chronoscope, the 

 time which elapses between the commencement of a flash of lightning and a 

 motion of the hand made in consequence of perceiving it. In the case 

 of one young man, whom I will denote by M, I had found this in- 

 terval to be 0'145 of a second. 



Ihave recently repeated these experiments in the case of two of 

 my pupils, and have found that this interval amounted in one case 

 to 0'183 and in another to 0-165 of a second. 



II. During the late frost I incited one of my pupils to make ex- 

 periments to determine the coefficient of the friction of iron upon ice. 

 The young man remained standing on his skates (which were not 

 grooved) holding in his hand a spring balance. To this a stout cord 

 was fixed by which a second skater drew him along the ice. At 

 the commencement the balance indicated a pull of from 10 to 12 

 pounds ; during the course it varied between 2 and 4 pounds. As 

 the subject of the experiment weighed 125 pounds, the coefficient of 

 friction in question amounted at the commencement to 8-9 percent, 

 of the burden, but during the motion only to 16-3'2 per cent. — Pog- 

 gendorff's Annalen, March 1870. 



Freiburg im Br , January 1870. 



2 H 2 



