390 THE POPULAR SCIENCE MONTHLY. 



Having made and graduated both a direct alcohol and a differ- 

 ential air thermometer, the absolute expansions of water and alco- 

 hol are determined. Very accurate results may easily be got as 

 to the latent heats of water and steam. Then the student, having 

 made his calorimeter, determines the specific heats of iron, copper, 

 zinc, tin, and lead. The specific heats of a few liquids are deter- 

 mined either by direct comparison with water or indirectly with 

 the metals. 



In light, the chief work consists of the following : The making 

 and use of the diaphanous and shadow photometers ; the making 

 of an instrument for examining the rules of reflection and refrac- 

 tion, and the verification of these rules ; the determination of 

 refractive indices of liquids and their dispersive powers; the 

 images from curved mirrors, the measurement of focal lengths, 

 and the curvative and refractive indices of lenses. A few experi- 

 ments concerning plane polarized light are followed by the de- 

 termination of the wave-length by a grating, and the construction 

 and use of the spectroscope. 



The principal pieces of apparatus constructed for work in elec- 

 tricity are : A gold leaf electroscope ; a differential condenser ; a 

 sand-dropping accumulator ; a Leyden jar ; an electrophorus ; a 

 dry pile ; a voltaic cell ; a differential galvanometer ; a resistance 

 bridge ; a set of resistance coils ; a tangent galvanometer ; a po- 

 tentiometer ; a thermo-element ; a thermopile. And by these ap- 

 paratus typical experiments and measurements, of which the fol- 

 lowing are a few, are made : The study of magnetic curves ; the 

 action of the current on the needle ; the relation between length, 

 weight, and resistance in wires ; the effect of temperature on re- 

 sistance ; the law of divided circuits ; specific resistance ; electro- 

 motive force ; internal resistance of cells, and so on. 



Electricity, especially voltaic, lends itself perhaps more abun- 

 dantly to exact measurements in the elementary laboratory than 

 the other branches, and it is on this account, and because it is the 

 last subject treated of, and so claims any spare time at the end of 

 the term, that it occupies a rather prominent part. I do not hold 

 that it has really any greater educational value than the other 

 branches, and certainly in a general educational course it is not 

 for me to give it prominence, because just now it has a consider- 

 able technical development. I trust the time may never come 

 when any branch of physics will be considered as of compara- 

 tively little importance in general education. 



To-day I have particularized the method of teaching one 

 branch of science. I have had to use strong language, for I feel 

 strongly, and I have been addressing strong people. Of this, at 

 least, you and all men may be well assured, that I will not cease 

 to proclaim, as long as strength is given to me, that the hope of 



