80 CHEMICAL DISCOVERY AND INVENTION 



lift it. These can be lowered when required by means of the 

 curved brass wire, g, connected as shown in the figure with the 

 upright brass support. The wire is controlled by an excentric 

 cam, o, rotated by a handle passing air-tight through a plug in 

 the side of the case. 



Such a description as the foregoing is only capable of giving 

 an idea of the way in which the pneumatic principle is applied 

 to the determination of weight. As to the possibilities of such a 

 balance the statement of the authors is as follows : " Weights 

 of the order of one-hundredth of a milligramme may be com- 

 pared with the standard measures with an accuracy of one five- 

 hundredth of their amounts, i.e. the absolute value of such 

 weights can be determined with certainty to one fifty-thousandth 

 of a milligramme (2xlO- 8 gramme), while changes of weight 

 can be measured of an order as low as one two-hundred and 

 fifty-thousandth of a milligramme." 



With such appliances the mote in the sunbeam becomes a 

 ponderable mass ! 



Heating and Cooling. The common source of heat for the 

 purpose of ordinary experiment in the chemical laboratory is 

 the combustion of coal-gas, generally in the Bunsen burner, or 

 some modification of that familiar instrument in which the gas 

 is mixed with sufficient air to secure complete and smokeless 

 combustion. 



As nearly everyone knows a much hotter flame is obtainable 

 if the air is replaced by unmixed oxygen. The oxyhydrogen or 

 oxy-eoal-gas flame has long been used for special purposes, such 

 as the production of the well-known limelight employed in the 

 magic lantern and for theatrical purposes. Such a flame is also 

 capable of piercing a sheet of iron, if not too thick, and may be 

 used even for cutting armour plate. 



The temperature of the oxyhydrogen flame is in the neigh- 

 bourhood of 2000 C., but a still hotter flame is produced when, 

 in place of hydrogen, acetylene is used. The oxyacetylene flame 

 may indeed be used for cutting armour plate six inches in thick- 

 ness, at a rate more rapid than that of the saw. This, however, 

 is not a laboratory operation. 



The use of electric heating for warming houses and cooking 

 has been introduced long ago, but the progress actually made 

 is comparatively slow, owing chiefly to the cost. This method 



