302 LECTURES TO SCIENCE TEACHERS. 



to which we give the name of sulphurous acid. In order to 

 obtain sulphuric acid (H 2 S0 4 ), by the combustion of sulphur, 

 we must in some way or other add on to the dioxide a 

 third atom of oxygon, and form the body known as sulphur 

 trioxide (S0 3 ), and this, when it comes together with water, 

 forms sulphur^ acid, as we see by the equation 



S0 3 + H,0 = H,S0 4 . 



The problem, then, which we have to solve is how to get the 

 sulphur dioxide to take up this additional dose of oxygen. 

 There are several ways in which this may be done. I hold 

 in my hand some finely-divided metallic platinum. I place 

 some of the spongy metal in this tube, which I can warm 

 with the lamp, and over this platinum I will pass a current 

 of dry sulphur dioxide, mixed with dry oxygen gas. The 

 former gas is evolved in the flask (a), and the oxygen passes 

 in from a gas-holder through the tube (b). You observe 

 that before reaching the platinum these gases retain their 

 transparency, but that after passing over the metallic- 

 powder a dense white fume fills the tube and receiver. 

 These white fumes consist of sulphur trioxide, and we only 

 need to pass these fumes into water to obtain sulphuric 

 acid. In this reaction the platinum remains unaltered, it 

 simply enables the oxygen and the sulphur dioxide to com- 

 bine together. This, you will say, is a very simple mode of 

 making sulphuric acid, and one capable of being employed 

 for the production of the substance on any scale. Unfor- 

 tunately, however, this process does not succeed on a large 

 manufacturing scale, for in practice the platinum becomes 

 covered with dust and dirt, and its pores get stopped up, so 

 that after a while it no longer fulfils its function, and is 

 unable to bring about the required combination. 



There is, therefore, nothing left but for us to look out 

 for some other mode of effecting our purpose. It has long 

 been known, as I have said, that when a mixture of sulphur 

 and nitre is deflagrated, sulphuric acid is formed. This old 

 observation serves as the key with which to solve our diffi- 

 culty, and contains the principle upon which the modern 

 manufacture of sulphuric acid is founded. 



The apparatus which is before you illustrates in a 

 satisfactory manner the chemical reactions which take place 



