August, 1904.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



175 



Now, according to Professor R. W. Wood, the CI:iy- 

 dcn cflfcot does not depend on wave-length, liiit only on 

 time and intensity. It is, however, quite likely that the 

 red screen prevented a feeble general illumination of 

 the plate, and it appears possible that in this way it 

 stopped a certain reversing effect, and so caused the 

 plate to appear more sensitive to the red light. The 

 various results produced by different causes, as detailed 

 above, justify such a suggestion. 



What is Rezcrscif .' — One may well ask what is it that 

 is reversed and what is it that is changed in all these 

 experiments? Silver bromide, silver iodide, gelatine, 

 and often potassium bromide are present in the film. 

 It has been suggested, I believe, that there is some 

 occult combination between the gelatine and the silver 

 salts, but there does not appear to be any evidence to 

 support this notion. It has long been known that 

 silver iodide retards the reversal of silver bromide, and 

 this is probably the chief if not the only reason for its 

 introduction intti commeriial plates. If all the experi- 

 nients on exposure-effect and reversal re- 

 ferred to were made on plates of known 

 composition, including simple gelatino- 

 bromide films, we should certainly get more 

 information than we have. .A given com- 

 mercial plate is not always the same. All 

 makers strive to improve, and improvement 

 means change. I think, too, that there is 

 very good reason for doubting Professor 

 Wood's time-limit of about one-thousandth 

 of a second for what he calls the " light- 

 shock " effect. Different illuminants and 

 different plates might give other time- 

 limits. My own results that I described 

 last month, obtained by the use of Wynne's 

 shutter-speed tester, indicate that a modi- 

 fication or amplification of Professor 

 Wood's deductions from his experiments is 

 necessary. All the experiments and results 

 here referred to must be regarded as f)nly 

 initiatory. 



Ever-Set Shutters. — An "ever-set" shutter 

 has the obvious advantage that it is always 

 ready for use, and that is really the mean- 

 ing of the word invariably used to describe 

 such apparatus. It would, however, be 

 rather more correct to describe them as 

 never-set, for when an exposure is made the 

 shutter is not released, but the whole move- 

 ment of the parts that move is effected by 

 pressing the trigger or pinching the ball. The 

 closing is generally effected by springs, in which 

 case the opening has to be done against the pull of 

 these springs. Other things being equal, therefore, an 

 " ever-set " shutter requires more force to operate it 

 than one that is set by a separate operation and merely 

 released for the exposure. If the camera is held in the 

 hand this extra force required means more risk of 

 movement, and if a pneumatic ball is used when the 

 india-rubber or its connections are in a poor condition, 

 it means more risk of failure to operate the shutter. 

 Rut as is general in such cases, it would be wrong on 

 this account to condemn " ever-set " shutters as a 

 class, for as a matter of fact some of them require very 

 little force to operate them, less, probably, than the 

 force needed to release some of the shutters that are 

 " set " by a separate movement. But the fact re- 

 mains that the same shutter working under the same 

 conditions will need less force to release it when it is 

 previously set than to operate it without the previous 

 setting. 



Pea^t and its Mode of 

 FormaLtion. 



By F. E. I''RiTstH, n.Sc, Ph.D. 



Whenever plant-remains are deptvsiteti at a rate which 

 exceeds the rate of decomposition, we get a mass of 

 semi-decayetl \egetable substaiure of a brownish or 

 blackish colour and of soft consistency, which we call 

 peat. Several conditions are necessary to admit of such 

 a deposit being formed, the most important being in- 

 sufficient drajnag'e (i.e., accumulation of more water 

 than is removed) and lack of the ordinary rapid ag'ents 

 of decomposition. The silting up of a river or of a 

 lake, or the destruction of an area of forest, may both 

 lead to the formation of a swamp, which gradually b'^ 

 comes firmer and drier bv the advent of marsh-plants; 



Upland Wood. Heather and Bilberry Krowing in Peat formed in the Wood. 



as their remains accumulate a layer of peat is slowly 

 formed. Under ordinary circumstances all plant-re- 

 mains are rapidly dex;onipnse(l by tiie vario^us organisms 

 occurring in the soil — ^mainly bacteria, but moulds and 

 a number of animals also take some part in the process. 

 This det~ay is especially rapid in warmer climates, inas- 

 much a.s the growth and activity of the above-mentioned 

 organisms is much increased at higher temperatures, 

 and, consequently, peat is almost unknown in the 

 tropics. In colder regions, however, an abundant 

 vegetation, together with incompetent drainage, is 

 generally too much for the agents of decomposition 

 and, when a layer of peat has once originated in this 

 way, the process goes on more and more rapidly owing 

 toi the antiseptic action of the organic acids, formed by 

 the, part i;d de<;ay of the vegetable substance. 



Mr. C. E. .Moss has recently published an interesting 

 study of the peat moors of the F'ennines. A large 

 portion of the summits and slopes are occupied by such 

 moors, which are quite wanting, however, on the steeper 

 slopes, leading down to the lowlands. Three different 



