ON EUDIOMETERS. 285 



that we owe the greatest advance in the analysis of gases. 

 We have on the table a fairly complete collection of 

 Bunsen's apparatus, supplied by Desaga of Heidelberg. 

 One of the instruments is what is termed an absorption 

 tube, or absorption eudiometer. It consists of a tube about 

 250 millimetres in length and twenty in diameter. In this 

 the absorption of different gases by reagents is carried out. 

 Besides that you have an explosion eudiometer which is very 

 much longer 800 millimetres in length provided at the 

 top with a pair of platinum wires, by means of which an 

 electric spark can be passed through a mixture contained in 

 it. The tubes are first graduated, but they are not divided, 

 as was generally the custom in previous times, into certain 

 volumes, but into lengths. They are divided into milli- 

 metres, and the apparatus by which the division is made 

 is on the table. It consists of a double board with a 

 graduated scale fixed on one side, and the tube to be 

 graduated is placed at the other end. The tube is covered 

 with wax, preferably mixed with a small quantity of 

 turpentine. This is finely spread by means of a brush 

 over the glass tube, which is then placed at the end of the 

 apparatus. It is covered with two pieces of metal, one of 

 which has a perfectly straight edge and the other has an 

 edge with notches in at it every five millimetres, giving 

 the long lines of the divisions. In order to graduate you 

 use what is virtually a pair of beam compasses, a rod 

 of wood with a point at one end and a knife-edge at the 

 other. The point is placed in a scale, and by means of the 

 knife-edge you make a scratch on the wax, and by this, in 

 a short time, you get a tube of 800 millimetres gra- 

 duated. After you have scratched the wax it is necessary 

 to see that you have made no mistakes before the tube is 

 removed. After having satisfied yourself that there are 

 no mistakes, or correcting them by melting the wax by 

 means of a red-hot platinum wire and re-marking it, the 

 tube is removed, after the numbers are marked, by means 

 of a steel pen, because that gives a double line in the 

 down-stroke, which makes the graduation more easy to 

 read. The tube is then placed over a trough of lead, con- 

 taining a mixture of sulphuric acid and calcic fluoride 

 the hydrofluoric acid which passes off, etches the tube very 

 evenly, and gives you divisions which are easily read. I 



