ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



679 



of copper, slightly less than 1/16 in. thick, 18 in. long, and 10 in, wide at 

 one end and running to a sharp point at the other, as shown at 8 in fig. 190, 

 is supported horizontally upon two legs at the wide end, and at some 

 distance from the pointed end by another leg, these three legs constituting 

 a firm tripod base for the whole device. Under the pointed end of the tri- 

 angular plate of copper is placed a small Bunsen gas-burner, with an 

 aperture of about 1/8 in., and connected with the gas-supply of the 

 building by means of a rubber tube. If the flame is allowed to burn 



steadily at about half its full force, and permitted to play upon the copper 

 plate at a distance of about 1 in. from its extreme point, as shown in the 

 figure, the whole plate will soon be heated, but the temperature will be 

 found to gradually diminish towards the wide end. At a distance of about 

 12 to 13 in. from the point where the flame acts upon the copper plate the 

 temperature will remain steadily at about 56° C, with the temperature of 

 the room at 22° C. As long as the temperature of the room remains nearly 

 the same the temperature of the plate at any given distance from the burner 

 will also remain at the same point. This constancy is due to the fact that 

 the heat which is conducted through the copper plate with constant 

 rapidity from its source — the burner — is radiated into the surrounding air 

 at an equally constant rate, and as one passes towards the wide end of the 

 plate from the burner, trials with the thermometer show that there may be 

 found an infinite number of points in succession at which the temperature 

 is very nearly constant. 



In order to use the paraffin itself as an indicator of the proper tempe- 

 rature, and in that way dispense with the thermometer altogether if desir- 

 able, it was necessary to use a new type of cup in which to melt the paraffin. 

 The paraffin-cup or trough p shown in the figure is made of copper, tin-lined, 

 and is 6 in. long, 1^ in. wide, and Ij in. deep. In practice the cup is half 

 filled with paraffin and placed lengthwise on the copper plate, with its 

 narrowest side towards the flame, and about 9 in. from it, as shown in the 

 cut. Tlie paraffin-cup may be covered with a slip of glass to exclude dust. 

 If the burner plays upon the plate as directed, and the trough is in the 

 proper position, in about an hour it will be found that the paraffin in the 

 trough has been melted at the end nearest the burner but has remained 

 congealed at the other. Moreover, it will be found that the point 

 where the melted comes in contact with the nearly frozen paraffin is 

 very constant, and it is just at this point where it is safe to place objects 

 which are to be imbedded. The paraffin which remains congealed in the 

 trough is represented in the cut by the shading at the remote end of the 

 trough, the clear space below the dotted lines nearest the flame indicating 

 the portion which remains molten. 



