520 Dr. Barker on the relative value of the 



rent that the test-papers of Schonbein and Moffat varied con- 

 siderably in susceptibility of colouring, a formal course of com- 

 parative observations was commenced in November 1854, and 

 has been continued to the present time. 



Other observers have instituted comparative trials of the two 

 ozone papers; but the extent, as well as the numerical precision, 

 of the results obtained at Bedford must render them valuable 

 to all who wish to employ the utmost precaution in observations 

 on ozone. 



In order that the two papers might be submitted to precisely 

 the same test, a box was constructed on the plan recommended 

 by Mr. Prince of Uckfield*. It freely admitted the air, without 

 a ray of light. Schcinbein's papers were procured from Mr. 

 John Cox of Rye Lane, Peckham (who was, for some time, the 

 accredited agent for both the test-papers), the others were 

 received from Dr. Moffat himself. The two kinds of test-paper 

 were suspended, side by side, in the centre of the box, and, to 

 avoid any accidental failure of a slip, two of each kind were 

 used. Observations were recorded daily at 9 a.m., not oftenei', 

 except at certain times, when ozone was present in more than 

 its average quantity, and the papers were found highly dis-- 

 coloured at 3 p.m., the ordinary hour for meteorological obser- 

 vations. The papers were changed only when they had become 

 tinged. At various times they were left during intervals vary- 

 ing from one to seventeen days without suffering any change of 

 colour. 



In most instances the discoloured papers were compared with 

 Dr. Moffat's scale of brown tints ; but sometimes, to vary the 

 mode of testing, they were immersed in water according to 

 Schonbein's instructions. In each process the results were 

 relatively the same. Thus, if a coloured paper agreed with 

 No. 5 of Dr. Moffat's brown tints, before immersion, it also 

 agreed with No. 5 of Schonbein's scale of purple tints, after 

 immersion. 



With these explanations of the carefulness and fairness of the 

 observations made at Bedford, we now lay before our readers 

 the following table of the results obtained : — 



* Its construction is described and figured in a paper given in the ' As- 

 sociation Medical Journal,' March 3, 1854. 



