Ozonometers of Drs. Schonbein and Moffat. 519 



that before we can safely say anything respecting its probable 

 effects, we must first be sure of its presence or its absence. 

 Nothing could be more mortiiying to a medical observer than 

 to find, after he had explained certain phjenomena as connected 

 with the absence of ozone, that this principle was in fact present 

 at those times when the said phenomena occurred. To prevent 

 such an error, we must have the best possible test-papers or 

 ozonometers. 



For the benefit of readers who may require such information/ 

 we may briefly explain here, that the ozonometer consists simply 

 of strips of paper prepared with iodide of potassium and starch. 

 These papers are suspended so as to be exposed to the free access 

 of air, but not to the direct rays of the sun. The paper, when 

 affected by ozone, is found tinged with various shades of brown, 

 of which the intensity is measured by a scale of ten gradations. 

 Dr. Schonbein recommends that test-paper, prepared according 

 to his own formula, should be suspended in a spot to which the 

 air, but not the direct rays of light, may have free access, and 

 that it should be removed from the neighbourhood of stables, 

 manure-heaps, &c., where the gases generated might vitiate the 

 observations. The brown tinge of the ozonometer is produced 

 by the decomposition of the iodide of potassium — the oxygen of 

 the ozone (which Faraday defines as oxygen in an allotropic 

 condition) combining with the potassium, and setting free the 

 iodine, which now forms iodide of starch. In its dry state this 

 new combination is of a brown colour; but when moistened 

 with water it assumes a blue tint, of which the intensity indi- 

 cates the quantity of ozone. This test-paper becomes colom-less 

 shortly after its immersion in water. 



The next ozonometer of which we have to speak is the paper 

 prepared by the formula of Dr. Moffat of Hawarden, the fii'st of 

 ozoue observers in our own country. His recorded observations 

 commenced in 1848. In his directions for the use of his own 

 ozonometer, Dr. Moffat requires that the test-paper be suspended 

 in a box, so perforated as to admit a free passage of air, but not 

 of light. When thus exposed to the action of air containing 

 ozone, the prepared paper acquires a brown tinge, varying in 

 intensity from 0° to 10°. Dr. Moffat's plan does not require 

 the moistening of the paper to procure the blue tint. Provided 

 it be kept in the dark, his test-paper will retain its brown tints 

 for a long period, even for two or three years. 



AVe have now to present to our readers the results of a series 

 of careful comparative observations on these two test-papers for 

 ozone. The observations made at Bedford commenced, in the 

 first instance, in October ] 853, and when it had become appa- 



