226 Prof. Draper's Description of the Tithonometer. 



A stout tube, six inches long and one tenth of an inch interior 

 diameter, ef is now fused on at c. Its lower end opens into the 

 main siphon tube; its upper end is turned over at/, and is nar- 

 rowed to a fine termination, so as barely to admit a pin, but is not 

 closed. This serves to keep out dust, and in case of a little acid 

 passing out, it does not flow over the scale and deface the divis- 

 ions. At the back of this tube a scale is placed, divided into 

 tenths of an inch, being numbered from above downwards. Fifty 

 of these divisions are as many as will be required. Fig. 2 shows 

 the termination of the narrow tube bent over the scale. 



From a point one fourth of an inch above the stage d, down- 

 wards beyond the bend, and to within half an inch of the wire 

 z, the whole tube is carefully painted with India ink so as to 

 allow no light to pass ; but all the space from a fourth of an inch 

 above the stage d to the top of the tube a, is kept as clear and 

 transparent as possible. This portion constitutes the sentient 

 part of the instrument. A light metallic or pasteboard cap, A D, 

 fig. 3, closed at the top and open at the bottom, three inches long 

 and six tenths of an inch in diameter, blackened on its interior, 

 may be dropped over this sentient tube ; it being the office of the 

 stage d to receive the lower end of the cap when it is dropped on 

 the tube so as to shut out the light. 



The foot of the instrument k I is of brass ; it screws into the 

 hemispherical block m, which may be made of hard wood or 

 ivory ; in this three holes, p q r, are made to serve as mercury 

 cups ; they should be deep and of small diameter, that the metal 

 may not flow out when it inclines for the purpose of transferring. 

 A brass cylindrical cover, L M, L M, may be put over the whole ; 

 when it is desirable to preserve it in total darkness, it should be 

 blackened without. 



Secondly, of the fluid part. — The fluid from which the mixture 

 of chlorine and hydrogen is evolved, and by which it is confined, 

 is yellow commercial muriatic acid, holding such a quantity of 

 chlorine in solution that it exerts no action on the mixed gases as 

 they are produced. From the mode of its preparation it always 

 contains a certain quantity of chloride of platina, which gives it 

 a deep golden color, a condition of considerable incidental impor- 

 tance. 



When muriatic acid is decomposed by voltaic electricity, its 

 chlorine is not evolved, but is taken up in very large quantity and 



