186 SCIOPTICON MANUAL. 



water on top is run off by means of an india-rubber tube, 

 acting as a siphon, the weights replaced on the pan, and 

 the holder is ready for refilling. 



" Where the oxy-hydrogen light is much used, holders are 

 almost indispensable, and in all cases effect a large saving of 

 time and material in the preparation and use of the gases." 



The above illustrated article has been prepared for the 

 purpose of affording a practical guide in constructing similar 

 gas-holders, wherever the plan is approved. If the tubs 

 are made at any distance away, they can be brought in parts 

 to the place they are to occupy and there set up. The joints 

 of the galvanized iron holders are made gas-tight with solder. 



Wherever illuminating gas is in use, the hydrogen gas 

 holder can be filled with it, as is done at the Franklin 

 Institute. If either the alcohol burner, or the concentric 

 jet with house gas direct from the main is used, only an 

 oxygen gas holder is required. 



So far as we now know, it seems best to generate the gases 

 by repeated use of the apparatus described on pages 165 

 and 167. With two retorts, one accustomed to the operation 

 can roast a double oxygen charge in each alternately till 

 eight pounds are used, which about fills a gas holder of the 

 dimensions described. 



The use of gas holders renders projecting apparatus much 

 more available in educational institutions, because the gases 

 can be made in large quantities at once, stored for an in- 

 definite time without deterioration, and used whenever 

 occasion requires, without delay, with only such darkening 

 as can be readily effected, either at evening or in the daytime. 



OTHER GAS CONTRIVANCES. 



A desire to render the lime-light apparatus more conven- 

 ient, inexpensive, safe and portable, gives rise to various 



