CONSTRUCTION OF THE INSTRUMENT. 393 



scale is placed, divided into tenths of an inch, being numbered from above downward. 

 Fifty of these divisions are as many as will be required. Fig. 106 shows the termi- 

 nation of the narrow tube bent over the scale. 



864. From a point one fourth of an inch above the stage, d, downward 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 paste- 

 board cap, A D (fig. 107), 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. 



865. 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 is inclined for the purpose of transferring. A brass cylin- 

 drical 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 within. 



866. 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 colour, a condition 

 of considerable incidental importance. 



867. When muriatic acid is decomposed by voltaic electricity, its chlorine is not 

 evolved, but is taken up in very large quantity and held in solution ; perhaps a bichlo- 

 ride of hydrogen results. If through such a solution hydrogen gas is passed in minute 

 bubbles, it removes with it a certain portion of the chlorine. From this, therefore, it 

 is plain that muriatic acid thus decomposed will not yield equal measures of chlorine 

 and hydrogen, unless it has been previously impregnated with a certain volume of the 

 former gas. Nor is it possible to obtain that degree of saturation by voltaic action, no 

 matter how long the electrolysis is continued, if the hydrogen is allowed to pass through 

 the liquid. 



868. Practically, therefore, to obtain the tithonometric liquid, we are obliged to de- 

 compose commercial muriatic acid in a glass vessel, the positive electrode being at the 

 bottom of the vessel and the negative at the surface of the liquid. Under these circum- 

 stances, the chlorine, as it is disengaged, is rapidly taken up, and the hydrogen being 

 set free without its bubbles passing through the mass, the impregnation is carried to 

 the point required. 



869. Although this chlorinated muriatic acid cannot, of course, be kept in contact 

 with the platina wires without acting on them, the action is much slower than might 

 have been anticipated. I have examined the wires of tithonometers that have been in 

 active use for four months and could not perceive the platina sensibly destroyed It is 



BB 



