86 Transactions of the 



Paper stained with plumbic sulphide (PbS) is whitened by 

 the formation of the sulphate (PbSOJ. 



Silver leaf has lately been proposed as a test paper, but it 

 is not likely to lead to any reliable results. 



The best test is probably that devised by Dr. Van Huizuiga, 

 namely, paper soaked with protoxide of thallium (TlgO) ; the 

 action of ozone changes it brown, owing to the formation of 

 the sesquioxide (TI2O3). It is prepared by washing paper 

 with a solution obtained by precipitating thallous sulphate 

 (TI2SO4) by baric hydrate (BaHjOj). It is calculated that 

 on every square centimetre there is one millegramme of 

 thallous oxide. 



Ozonometry is at present very deficient, little being known 

 of the actual amount present in the atmosphere. Most of 

 the early observations on ozone in the atmosphere are vitiated 

 by the knowledge that due precautions were not taken to 

 ensure absence of mistake from the presence of other sub- 

 stances of like character. For the purposes of ozonoscopy, 

 at present an arbitrary scale is used from 1° to 10°, deter- 

 mined by variation of colour on the iodine starch paper pre- 

 pared by standard proportions. 



Ozone differs from oxygen in the following important par- 

 ticulars : — (1) It has a strong odour. (2) It is poisonous. 

 (3) It bleaches vegetable colours. (4) It liberates iodine 

 from potassic iodide. (5) It destroys by oxidation deleterious 

 miasmata. (6) Attacks silver, gold, and many difficultly oxi- 

 dizable metals and decomposes almost all organic compounds. 



Its resemblance to chlorine I have already traced. There 

 is also a striking analogy between it and sulphuric dioxide, 

 substances of the same molecular constitution; that is to 

 say, the molecule of each consists of three atoms of diatomic 

 elements similarly condensed. They both possess a peculiar 

 and characteristic odour — are colourless and dense; both 

 deodorize, disinfect, tarnish, and corrode moist metals, 

 whiten paper stained with plumbic sulphide, are absorbed 

 by a solution of potassic iodide, rendering the liquid yellow 

 from the presence of free iodine, and also by oil of turpen- 

 tine. Another modification of ozone, antozone, combines 

 with water 



to form (peroxide of hydrogen) a liquid partaking of the pro- 

 perties of ozone, while sulphuric dioxide, when combined with 

 water, also produces a liquid (sulphurous acid (II2SO3) pos- 

 sessing the properties of the gas ; ozone reduces silver from 



