Faraday to Jacob Herbert   10 February 1845

Royal Institution | 10 Feby 1845

Dear Sir

Mr. Wilkins has placed in my hands four pieces of plate glass which I will number & mark according to their thickness[.]

No 1. Cooksons 0.52 of an inch thick

2. British Company - 0.54 of inch in thickness

3. Thames Company - 0.55 <-> thick triangular

4. D˚ <-> - 0.60 <-> thick Square

Of these 4 & 3 have the least colour - the colour being a pale green. 2 and 1 are much deeper in colour - 2 being of a greenish blue colour & 1 green[.]

An idea of the effect of this colour when the glass is used for windows may easily be obtained by placing a large sheet of white cartridge paper on a table near a window so as to be uniformly illuminated & then laying the pieces of glass down upon it and observing how much the paper under each piece of glass is altered in its appearance from that part which is left uncovered[.] It will quickly be seen that 1 & 2 lower the colour most and about equal to each other;- 3 & 4 the least. So superior in this respect are 3 & 4 to either of the others that if laid one on the other so as to give more than a double thickness, they do not stop more light than either of the others alone[.]

As these glasses differ from each other in colour and as in light houses it is the yellow light of a lamp that is used I have compared the glasses together in the same way by lamp light and as was to be expected the advantage was rather more in favour of 3 & 4 than by day light for the blue & bluish green tints of 1 & 2 are less favourable to the transmission of the rays of light from a lamp than the warmer tints of glasses 3 & 4.

In reply therefore to the question respecting the increase of thickness in the glass used it is I think important that if such glasses as 1 and 2 be used the thickness should not be increased beyond that which is quite necessary on other accounts; and also that if 3 & 4 were in other respects equally good they would allow a double thickness of glass to be used without involving a greater loss of light[.]

Three smaller pieces of glass have also been placed in my hands[.] No. 5. British - No. 6. Cooksons - No. 7. Thames. 5 is clear blue in colour - 6 green and 7 yellow. No. 7 has evidently been much more free from colour formerly but exposure to light has caused it to become yellow. If this should prove to be the only change however it would not matter for it still has less colour (as is seen when placed on white paper) than either of the other two & the colour it has is of less consequence to artificial or lamp light than the blue or green[.]

I have tried all these glasses to ascertain if I could detect any difference in their liability to be affected on the surface & roughened by the action of weather or salt spray but I cannot make any impression on any of them. I have also tried them as well as I could for hardness by rubbing them with emery powder one against the other but here again cannot distinguish a difference. As far as my tests apply they are in these respects nearly alike[.]

The force of a hurricane is estimated at somewhere about 32lbs on a square foot and when loaded with spray it may be more than this. I may perhaps be excused for observing that where plates of glass have to bear such power as this it is very important that they should be carefully bedded in their frames in the putty or cement employed so as to bear on all parts of the surface near the edge and not more or less upon one point than another¹[.]

I am | My dear Sir | Your Very faithful Servant | M. Faraday

Jacob Herbert Esq | &c &c &c