Use Mohs' hardness scale to answer the following question.
Mohs' Hardness Scale
softer
Talc
fingernail
(2.5)
Gypsum
2
Calcite
Fluorite
glass
(5.5)
Apatite
Orthoclase
Quartz
Topaz
8
steel nail
(6.5)
Corundum
9
Diamond
10
harder
A sample can be scratched by glass but not by a fingernail. Which two could be a sample be made of? A. Gypsum B. Fluorite C. Apatite D. Orthoclase

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.