284 LECTURES TO SCIENCE TEACHERS. 



described in the Philosophical Transactions. Here is one o 

 Pepys' eudiometers also belonging to the Dalton collection. 

 This contains one cubic inch of gas to the lowest division. 

 There is a bent tube carefully ground in, and fitting so that 

 when placed over the tube below there is no possibility of 

 loss of gas. The india-rubber bottle attached to the bent 

 tube is completely filled with an absorbent, and when so 

 filled it is inverted and the tube fixed into the eudiometer 

 which contains the gases under water or mercury. By 

 simply scmeezing the bottle you can introduce the reagent, 

 squirting it up the sides of the tube and causing absorption. 

 After the absorption is complete you may read off the 100th 

 divisions with ease, provided the surface of the water or 

 mercury is exactly at the division. But Pepys invented a 

 very ingenious mode by which one-tenth of one of these 

 small divisions might be read. The apparatus was a wide 

 cylinder with a small opening at the bottom closed by a 

 cork, through which a narrow glass tube drawn out to a 

 fine point was capable of being moved. This was connected 

 by means of a steel joint with an india-rubber bottle filled 

 with mercury or water generally mercury. The tube was 

 drawn down below the level of the mercury in the vessel, 

 then the eudiometer was placed over the narrow tube 

 and depressed, or the india-rubber bottle pushed up until 

 the point of the tube came just above the surface of 

 the mercury within the eudiometer. Then by opening the 

 stopcock a small quantity of liquid ran down into the 

 bottle below, and raised the quantity of mercury within 

 the eudiometer until it came exactly to the level of a 

 division. When this was done, a certain portion of gas 

 which corresponded to the fraction of the division had been 

 introduced into this narrow tube. The narrow tube was 

 graduated so that each division represented T ^th of the 

 smallest division of the eudiometer, so that the eudiometer 

 being divided into lOOths and this tube into T Vths of 

 those, it was possible to work to xoth of a per cent., and 

 Pepys' results agreed very accurately indeed with that 

 small fraction. 



A number of different absorbents have been used at 

 different times for oxygen and other gases. The alkaline 

 pyrogallate has been employed, and I shall presently show 

 you an apparatus in which it is used. But it is to Bunsen 



