Research on some Standards of Light. 



487 



The best method of preparing the lime is to ignite calcium carbonate 

 which has been precipitated from calcium nitrate by ammonium 

 carbonate.* 



3. The hydrogen burned must contain no hydrocarbons. 



Afc these high temperatures platinum combines readily with carbon, 

 and any carbon in the flame would rapidly tarnish the surface of the 

 metal. 



4. The gases should be burnt in the ratio of four volumes of 

 hydrogen to three of oxygen. 



All the best results were obtained when using the gases in the above- 

 mentioned proportion. The temperature of the flame is then but 

 little above the melting point of platinum, and the metal does not 

 superheat to any great extent. When the platinum is considerably 

 superheated it slowly distils, the drops condensing on the brick forming 

 the cover of the furnace ; the metal then drips back into the crucible 

 carrying with it some of the silica which the brick contains, and thus 

 contaminating the surface which is under observation. An excess of 

 oxygen in the flame is also favourable, inasmuch as it oxidises any 

 impurity that the platinum may contain. 



Finally it is important that the blow-pipe should be so constructed as 

 to ensure a thorough mixture of the two gases. 



The most favourable conditions being established by these prelimi- 

 nary experiments, the necessary apparatus had now to be designed. It 

 may be thought that the instruments are unnecessarily complicated, 

 but it must be borne in mind that a single experimenter had not only 

 to make all the observations, but also to regulate the flow of the gases, 

 the electromotive force on the terminals of the standard lamp, and the 

 current of water passing through the diaphragms. Most of the appa- 

 ratus had, therefore, to be made automatic in its action. 



The instruments serving to regulate the supply of gas are shown 

 in fig. 6. The gas supply is stored in large cylinders at a pressure 

 of from 100 to 200 atmospheres. S is a device worked by an 

 electric current from a key placed under the photometer head. When 

 the key is pressed the chopper C falls, compressing the india-rubber 

 tubes and cutting oft* the gas from the blow-pipe. (This device is shown 

 in elevation at the bottom of fig. 6, on the left-hand side.) The 

 hydrogen escapes through a valve to the burner B where it is ignited, 

 the oxygen blows off through the water trap T. When the chopper 

 C is up the gases, after passing through the meters provided with 

 scales reading to one-hundredth of a cubic foot are burnt from the 

 blow-pipe P. 



On one of the circuits the gas is forced through a diaphragm with 

 an opening of about one-hundredth of the normal cross-section of the 



* This method was suggested by Dr. A. Scott, to whose kind advice the solution 

 of many of the chemical problems involved in the present work is due. 



