CAS Registry number    All chemicals have a CAS (Chemical
Abstracts Service) registry number. Each and every chemical has
a number which is usually unique.  This is given in each case in
square brackets. If you have access to CAS on-line, or some
other search services, you can search for the element using the
CAS registry number. The registry number is a "smart number".
The last digit is a check-digit to which helps to ensure that
any given CAS number is authentic.   The way to check whether
the CAS number is authentic is as follows. Take the second digit
from the right and multiply it by 1. Take the third digit from
the right and multiply it by 2. Repeat this process for the
fourth (multiply by 3), fifth (multiply by 4), sixth digits
(multiply by 5) and so on for as many digits as there are. Now
add all the numbers resulting from the multiplications together.
Take the unit digit from this addition. It should match the last
digit of the CAS registry number. If not, the CAS number is
invalid.  As an example, c