til 



Hl.uW 1-1 1-K. 



lil.nW 1'II'K. 



portion 4, which rising as it were from out of the 

 cop produced by the bin*, ascends in U form of cone. "In C!OM 

 coSMeetyni with thie COM will be observed a smaller on* (a) contained 

 withm ^ of a dark colour, and rising from the upper extremity of the 



!.. . 



wick ; and by a very careful examination it will be found that the 

 outer xirface of the luminous cone is bounded by a thin coating uf a 

 (tightly luminous flauie (e 6), which form* the continuation of the blue 

 ring, and increase* a little in thickness as it approaches tin- up|>cr 

 extremity. 



The three cone* thiu enveloping each other differ not only in their 

 appearance, but aluo in their teni|>er.iture and chemical condition. 

 name, a wan ahown by Sir Humphry Davy in the course of his 

 beautiful and philosophical inquiries into its nature, which terminated 

 with the discovery of bin safety-lamp, is gaseous matter heated to 

 whitcueM: its most striking projicrties are evidently it* power of 

 communicating light and heat, and however closely these may appear 

 to be connected, the circuuutanceH l.y which the one may be developed 

 to its greatest extent in a flame are unfavourable to the production of 

 the other. The explanation of this U simple and obvious : the heat 

 depends on the rapidity and energy of the chemical combination* 

 taking place ; the light, on the contrary, on the quantity of the matter 

 kept at the white heat, and on the length of time it remains in that 

 Ute. If therefore into a stream of burning gas (to take a ]iorticular 

 ease, let it be coal gas) a jet of oxygen be conducted, the combuation 

 win be immediately rendered more rapid, the temperature of the flame 

 will consequently rise, while iu illuminating power diminixhes, as will 

 probably have been observed by many who have seen the oxyhydrogen 

 flames, where the light u derived only from the strongly heated chalk, 

 not from the burning gases. On applying these riewn to the common 

 flame, the existence of the three concentric cones will be readily under- 

 stood : in the exterior cone, the inflammable gases arising from the 

 decomposition of the burning material come in direct contact with the 

 atmosphere, are well supplied with oxygen, and they consequently here 

 undergo a more rapid combustion than the interior enclosed portions : 

 hers therefore will be found the hottest points of the flame. That 

 such is really the fact may be proved experimentally, by holding a 

 fine iron or platinum wire across the flame, when it will be found to 

 glow most strongly in the points of its emergence from the luminous 

 cone, and by holding the wire at different elevations in the flame, it 

 will be found that the portion of the outer cone immediately above e, 

 the upper ridge of the blue cup, is the point of greatest heat. Iu the 

 most luminous cone the combustion u slower, and in the interior 

 darker portion the gases have not yet come into contact with the air, 

 and are still unchanged. 



If a fine current of air be now directed into the flame by means of 

 the blow-pipe, it will assume the appearance seen iu /./. '2 : in the 

 centre of the flame, and immediately proceeding from the orifice . .1 Hit- 

 tube, a long and thin blue portion in the position <l e of the figui > v. ill 

 be seen ; this corresponds with the blue cup of the natural flame. But 

 it was in the upper edge of this cup. in which were found the points 

 of greatest beat, and the same U true here also, with this difference 

 however, that while in the natural flame these point* were spread over 

 considerable circle, e r, in the blow,pipe flame they are all collected 

 into the me noint t. where their united effect is of course projx.rtion- 

 ably gnat. The reason therefore of the high temperature which may 

 be produced by the blow-pipe is the result of the concentration of the 

 hottest points of the flame into a focus ; and another circumstance 

 tends also to heighten this effect, that while in the natural flame the 

 points of greatest heat are on its outer boundaries, and are therefore 

 rapidly robbed of their temperature, they here occur encased by the 

 luminous flame which thus protects them against the loss of teiiiix-ra- 

 tnre from this cause. 



The blow.pipe employed by the workman in the soldering of metals, 

 sod constructed an was first described, cannot be employed in these 

 operation., owing t.. the collection of the water from the condensed 

 I of the brrath on continuing the blast sny time. Tin. iuo.n 



is avoided by making the !.].. ].;.. ..i two pieces, ami l>y 

 interposing between these a receptacle for retaining the water, which 

 is thus prevented from entering into the finer part of the pipe where 

 it would obstruct the current of air. In using the blow pipe the ope- 

 rator must not employ his lungs in pr.l r, as it 

 would not only be detrimental to his health, but he would be un .!! 

 to sustain the blast a sufficient length of time to ensure the necessary 

 effects : it is produced by inflating the mouth with air, which . 

 forced through the tube by contracting the muscles of the cheek*, and 

 by a little practice the blast may be thus sustained for a considerable 

 time, the process of respiration being unaffected, the only inconvi 

 arising from the fatigue of the muscles of the cheeks from their uuu~n.il 

 exercise. The power of being able to perform thin dejiends on tin- 

 individual being able to keep his mouth inflated while he r> 

 After this has been learnt, some little experience will be required to 

 enable the operator to regulate the strength of the blast, so as t 

 duce the most powerful heat, as it must be neither too strong nor too 

 weak ; in the first case the heat is diminished in its action by an excess 

 of air, and in the second too feeble a flame U produced. 



We now proceed to the experiments themselves to which the blow- 

 |.i|K- may be applied, and we commence with those which fall under 

 the first clow : The changes produced on a body when exixwed Ml 

 high temperature. Of these, four are particularly worthy of n. 



Its fuwbility. 



The changes produced in \\M colour. 



The volatilisation of the substance under examination. 



The volatilisation of one or more of its component parts. 



When the various elements or their compoundx, which IK cm in . 

 solid form at the usual temperature, for these alone can here be con- 

 sidered, are exposed to heat, there is always evidence of a force tendin 

 to overcome that cohesion of their particle* to which they o\\ 

 solid form, and it is believed that by a sufficient degree of temperature 

 any Uxly whatever might be made to pass to the state of vapour. 

 either immediately or through the intermediate stage of fluidity. 

 However this may be, it is well known that the temperature nt which 

 such changes ore effected, varies with each element, and the point which 

 the l>li>w-pi]K> first informs us upon U, whether the body isone of those 

 which are unchanged or not at the degree of heat capable of being pro- 

 duced by means of it ; and according to the result we know among 

 what class of bodies the one under consideration will be fotuid. 

 is this mere fact the sole guide to the knowledge of the body under 

 examination ; the facility or difficulty with which the change is effected, 

 the characters of the substance in its changed form, the appearance it 

 assumes on being again allowed to cool. o|x>n to us new sources of 

 information, and each must be carefully observed. Thus, in some 

 minerals the fusion is produced with ease ; ill others again, it can only 

 be effected slowly, and by the strongest heat we can produce ; while in 

 a third cane, our efforts will only lie, sufficient to round off the sharp 

 edge of a fine fragment. 



Hut these are by no means the most important ilianges, the relations 

 of the elements to oxygen gas being decidedly more interesting and 

 instructive. When any substance combines with oxygen gas it is said 

 to be oxidised, and when a compound of oxygen with any base loses 

 oxygen, it is said to be deoxidised, or reduced to a lower state of 

 oxidation, according as it has lost the whole nr a jiart of its oxygen. 

 Most bodies, and particularly all the metals, are capable of undergoing 

 tin one or the other of these changes ; and as by means of the blow- 

 pi|- we have it in our power to produce at pleasure the condition* 

 under which a metal is liable to be oxidised, as well as those win 

 favourable to its reduction, should it be present in the form of an 

 oxide ; and as these changes are usually accomjuuiied with striking and 

 characteristic phenomena, the blow-pipe is thus the most powerful 

 instrument in detecting the presence of metals, which may in many 

 coses be extracted in their perfect metallic form from the smallext 

 fragment of their 



The oxidation will be produced by holding the Imdy before the 

 outer extremity of the flame, where the elements being heated in 

 contact with the oxygen of the air, are placed in the most favourable 

 circumstances for combining with it. This takes place the 

 readily the further the assay is held from the flame, provided a suffi- 

 cient temperature is at the same time obtained ; nor is it necessary 

 that this should be very great, since too great a heat is disadvantageous, 

 particularly when the support is of charcoal. This process will be best 

 performed with a pipe of comparatively large orifice, and when the 

 material is kept at a low red heat. 



The deoxidation or reduction requires a small orifice, and the sub- 

 stance under examination should be as much a* possible surrounded by 

 the luminous flame, by which means it is cut nil' from contact with the 

 atmospheric oxygen, and is surrounded with a glowing combustible ga-t, 

 by which it i deprived of its oxygen. In ]x-rfonning this operation, 

 which is infinitely more difficult than that of oxidation, particular 

 attention must be paid to keep the assay constantly in the luminous 

 flame, as the action is but little assisted by the charcoal on which the 

 substance rests. Bereelius recommends the beginner to practise him- 

 self in the reduction of metals by fusing small grains of tin on charcoal, 

 and to endeavour t<> keep it in that state without allowing its surface 

 to lose the metallic ghnee. wln.li it dm- owing to the formation of the 

 oxide, the instant it is removed from the deoxidising flame. Thin 



