APPENDIX A. 311 



carbon which is as sensitive to touch as a photographer's 

 plate is to light. 



7. A fixing fluid for carbon tracings. 



Gum damar 160 gms. 



Benzole q. s. ad 2000 c.c. 



If this solution be kept in a large museum jar in the 

 laboratory, the removed sheet bearing the tracings may be 

 dipped in toto or it may be subdivided and dipped in sec- 

 tions. Let the tracing be lowered quickly into the solu 

 tion and after a few seconds taken out and drained. If it 

 be now laid upon a sheet of filter paper or a newspaper 

 it will be dry in a few minutes. 



8. The cardiograph. 



Any laboratory will have different forms of cardio- 

 graphs for demonstration purposes, but not every labora- 

 tory is able to afford numerous duplicates. 

 An expert tinsmith will make the tam- 

 bour pans at very moderate cost, and the 

 student can do all the rest. Pans may 

 be made of two sizes No. 1, diameter 

 5 cm., depth 4 mm., outside diameter of 

 tube 3 to 4 mm., length of tube 3 to 4 cm. No. 2, dia- 

 meter 4 cm., depth 3 mm., tube as in No. 1, see Fig. 55, 

 To make the cardiograph : Take a tambour pan No. 1, 

 stretch thin sheet rubber the dentists' "rubber dam," and 

 sold as such by dealers across the pan and tie in place 

 with thread, A few drops of sealing wax will keep the 

 thread in place after it is tied. Mount the tambour as 

 follows : From any well seasoned, close-grained hard- 

 wood in boards, about 1 cm. thick, cut small triangular 

 pieces about 10 cm. on a side. In the center of each tri- 

 angle bore a hole to receive a medium sized cork (about 



