48 PROFESSOR J. W. MALLET ON THE STRUCTURE OF GOLD-LEAF, 



of the hammer falling in rapid succession upon different adjacent parts of the surface, 

 and partly to the lack of uniformity of support given by the other leaves above and 

 below in the packet. The view now stated receives confirmation from a point 

 strongly insisted upon by Mr. HANNA the very intelligent superintendent of the 

 W. H. Kemp Company's workshops namely, that for success in the gold-beating 

 process much depends on the condition of the animal membrane as to moisture or 

 dryness. If it be very dry the gold-leaf cracks or breaks, while if the membrane be 

 too moist the leaf sticks to it. The membrane requires to be dried or dampened to 

 correct the opposite effects of change in the atmosphere. This accords with the idea 

 that a certain amount of elastic stretching of the membrane, from which this 

 recovers, is necessary for the permanent or inelastic extension of the gold. In fact, 

 as the area of the gold-leaf is permanently extended by the beating, while that of 

 the membrane is not, the one film manifestly must slide over the other. It is 

 scarcely conceivable that this sliding shall occur at the moment at which a blow 

 falls, when friction between the surfaces is at a maximum. If not, it must occur 

 just afterwards, as a result of the elastic resilience of the membrane, Avhich leaves 

 behind it the plastic gold. 



It is evident that the statements to be found in the books as to the actual thickness 

 of gold-leaf based as they are upon weighing of measured areas represent only 

 averfif/e thickness, and that, in view of the decidedly greater thickness of these 

 microscopic threads of gold running through the mass than of the intervening parts, 

 the thickness of these latter parts must be notably less than the average. The 

 following determinations were carefully made with several square decimetres of leaf 

 in each case, accurately measured as to area, and weighed on a delicate assay balance. 

 The results are stated in " microns" (thousandths of a millimetre). 



Average thickness. 



Dentists' " fine " gold foil '9228/1 



In connection -with the microscopic examination of gold films by transmitted light, 

 it seemed to be interesting to make some observations on the absorption spectrum of 

 the metal, especially as there have been recently published the results of spectroscopic 

 study of the light which the metal reflects. 



It was proposed to examine for this purpose metallic gold in the following forms : 

 1. Pure or " fine" gold-leaf. 



