78 Dr. Meyer Wilderman on the Velocity of 



In case of freezing- or boiling-points being measured, a 

 simple incandescent-lamp can be used for getting the spot of 

 light on mirror and scale. As, however, the velocity of 

 reactions of a very great speed were to be measured, the 

 results were photographed by means of the arc. Two images 

 of the spot of light are then obtained from the glass and silver 

 surface of the mirror. One of these was suppressed by 

 sticking a properly chosen flat mirror to that of the galvano- 

 meter, and a lens of V\ metre focus to the glass of the 

 galvanometer. The silver mirror was protected from 

 moisture by placing a small dish with calcic chloride in the 

 galvanometer. 



Recording the Results. — The principle upon which measure- 

 ments of the velocity of reactions are based is, that while 

 a reaction of melting or separation of ice, or a reaction of 

 separation or solution of salt is going on, heat is evolved or 

 absorbed. If no heat is taken up by the system from the 

 surrounding medium, or given to it, the rise or fall of tem- 

 perature is a true indication of the amount of ice melted or 

 separated, &c. Advantage has been taken of the fact that 

 the variations of temperature can be speedily recorded by 

 the deflexion of the galvanometer. This enables us to 

 measure reactions of the greatest speed with certainty. 



A chronograph (chr.) of the best make of Richard and 

 Frere was procured. On the revolving drum " extra rapid " 

 bromide-paper was fixed, protected from light by the case 

 of the instrument. A thin horizontal slit, about 0*5 mm. wide, 

 20 cm. long, removed about 5 mm. from the bromide-paper, 

 allowed the light to pass to it. A very sensitive Breguet's fly 

 (with two springs) effectually regulated the clock, eliminating 

 small variations of the spring. The drum of the chronograph 

 is 60 cm. in circumference, about 20 cm. long, making 

 one revolution per minute (59J sec), so that 1 cm. indicates one 

 second. The drum can be stopped or set in motion at once. 

 To procure the best results, however, the clock should be 

 wound up before each experiment, and the recording only 

 begun after two or three revolutions of the drum have been 

 made, as no spring has the same power throughout the whole 

 length of its parts used. A wooden screen is placed before 

 the slit, which can be opened or shut at any moment, and 

 allows recording to begin at the proper time. 



To get the record true, the light of the arc is focussed on a 

 plate containing an adjustable vertical slit, whence the spot of 

 light passes to the mirror of the galvanometer and, reflected, 

 falls as a vertical line on the horizontal slit of the chronograph, 

 and is focussed on the bromide-paper. At first the zero- 



