August 2, 1888] 



NA TURE 



33. 



correct to call a piece of iron wire introduced into the sac of an 

 aneurysm to produce coagulation there, a fibrinogen. 



With regard, however, to these tissue-fibrinogens of Woold- 

 ridge, I think we may venture to offer a suggestion as to their 

 real nature, or, at any rate, as to the nature of one of their con- 

 stituents. From the last paper published by Wooldridge, we 

 find that they are imperfectly soluble in water, readily precipi- 

 tated by acids, and soluble in excess of those reagents ; that 

 they yield on gastric digestion a substance which is insoluble and 

 which is rich in phosphorus. From these details of their 

 properties, I think we may draw the conclusion, not that they 

 contain lecithin, as Wooldridge affirms, but that they belong to 

 the group of proteids described in the former part of this 

 paper under Hammarsten's name of nucleo-albumin. Nucleo- 

 albumins yield when poured into water a stringy precipitate 

 resembling mucin, and in a former paper Wooldridge speaks of 

 the precipitate of his tissue fibrinogen (precipitated by acetic 

 acid) as being a bulky one. If my conjecture is correct, it would 

 be exceedingly likely that when a saline solution of such a 

 substance was injected into the circulation, it would form strings 

 of a slimy mucinoid description in the vessels, and that these 

 would form the starting-point for the thrombosis or intravascular 

 coagulation that ensues. 



May 3. — " On the Induction of Electric Currents in Conducting 

 Shells of Small Thickness. " By S. H. Burbury. 



(1) Definition and Explanation of the Notation employed. — 

 A current-sheet in any field of electric currents is a surface to 

 which the stream-lines are everywhere tangential. A current- 

 sliell is the space between two current-sheets very near each 

 other. The superficial current in a current-shell is the quantity 

 of electricity which in unit time crosses unit length of a line 

 drawn on either sheet perpendicular to the current. If U, V, 

 W be the components of superficial current, there always exists 

 a function, <f>, called the current function, such that — 



dy dz 



I, m, n being the direction cosines of the normal. This function 

 completely determines the superficial currents. 



The corresponding expressions for the component currents per 

 unit of area are — 



dS d* dSd<l> „ 

 dz dy dy dz 



where S and * are any two functions of x, y, and z. 

 The components of vector potential due to a current-sheet 



F =//W/K"l-»£)- 



And if the sheet be closed, this may be put in the form — 



So that F, G, and II are linear functions of the <p's with 

 coefficient functions of the co-ordinates. 



If the current-sheet be spherical, the vector potential is 

 tangential to any concentric spherical surface. 



The electro-kinetic energy of a system of current-sheets is — 



2T = ft j (FU + GV + HWJrfS 

 over all the sheets ; that is — , 



///<♦(- i "•?)*>* 



if the surfaces be closed ; and if fl be the magnetic potential, 

 this reduces to — 



-//♦*« 



da. 



denoting the space variation of D. per unit length of the 



normal measured outwards. Also, , is shown not to be dis- 



dv 



continuous in passing through a sheet of superficial currents. 



T is expressible as a quadratic function of the $>'s with coefficients 



functions of the co-ordinates. 



(2) Comparison ruilh Magnetic Shells. — The components of 



vector potential due to a magnetic shell placed en a closed sur- 



face, S, with variable strength, <p (reckoned as positive when 

 the positive face is outwards), are — 



r.?//.t(-it-.i)l* 



They are, then, the same as those due to a system of currents on 

 S determined by <p as current function. Hence the magnetic 

 induction due to the magnetic shell is the same as that due to 

 the corresponding system of currents at any point in free space. 



(3) If fl denote the magnetic potential due to any magnetic 

 system outside of S, it is possible to determine <p so that a shell 

 of strength <f> on S has, at all points on or within S, potential 

 equal and opposite to n . General determination of <p to satisfy 

 this condition. The solution is unique. 



(4) Therefore, also, there exists a system of currents on S, 

 having <p for current function, such that the magnetic force due 

 to it is equal and opposite to that due to the external system at 

 all points on or within S. This system is called the magnetic 

 screen on S to the external system. Example of a sphere. 



(5) General Solution of the Problem of Induction, Resistance 

 not being yet taken into account. — If S , Cl Q , <p , Sec, relate to a 

 magnetic system outside of S, O. and <£ to S and superficial 

 currents upon it, the whole electro-kinetic energy is — 





dv 



dn 



dp 



(cin da\ 

 \dv dv) 



da' 

 avj 



d£i- 

 dv 



ws n 



dS. 



In this form, T has as many variables — namely, the values of 

 <f> — as it has degrees of freedom. 



If, therefore, the external system be continuously varied, the 

 induced current on S will be given by 



d 



dt 



that is, 



that is, 



■IT 

 d(p 



fdn dn\ 



\ dv dv) 



= o on S, 



= o on S, 



d fdn da\ 



dv(d? + lit) = 00nS - 



And since v 2 



«dCl 



and V 2 =0 at all points within S 



dt 



it follows that - ,~ + = o at all points within S. 



dn 

 dt 

 dd n 

 dJ 



That is, the induced currents, on their creation, are the mag- 

 netic screen to the time variation of the external field. This 

 gives the law of formation of the currents, however rapidly 

 they may decay by resistance. 



(6) Of a Solid Conductor. — If S be a hollow shell, there will, 

 as the direct result of induction, be zero magnetic force at all 

 points within it. Therefore, if it be filled with conducting 

 matter so as to form a solid conductor, none but superficial 

 currents will, as the direct consequence of the variation of the 

 external field, be induced in it. But as the superficial currents 

 decay by resistance, their variation induces currents in the inner 

 strata of the solid, so that in time, and no doubt generally in a 

 very short time, the solid becomes pervaded by currents. The 

 currents penetrate the solid, and the initial rale of penetration 

 can be calculated under certain conditions (see post, 15). 



(7) Of the Associated Function. — If F, G, H be the compo- 

 nents of any vector which satisfy 



</F 



dx 



dG 

 dy 



+ 



dR 



dz 



= o 



at all points within a closed surface, S, there exists a function, \x> 

 called the associated function, such that — 



*£, ' m IF + »;G + «H on S, 



dv 



V 2 X — ° within S. 



The components F, G, H of vector potential of a system of 

 closed currents outside of S have an associated function, x> on S. 

 dY _ <IG 

 dt ' dt 

 function, which shall be denoted by ty. 



In like manner - 



and - -, have 

 dt 



an associated 



