It is often valuable to know the degree of hardness of different substances, and these can easily be remembered by the following list. In degree of relative hardness the list is as follows, the hardest coming first.
| Diamond | Apatite |
| Corundum | Bell Metal |
| Topaz | Boric Acid |
| Quartz | Rock Salt |
| Iridium | Kaolin |
Take Kaolin as a basis. The number opposite each substance in the following list indicates its comparative degree of hardness in relation to Kaolin:
| 1 | Kaolin | TIE | See tie on Kaolin |
| 2 | Rock Salt | SNOW | Poured over Rock Salt |
| 3 | Boric Acid | HOME | Built of Boric Acid |
| 4 | Bell Metal | WIRE | Swinging a Bell |
| 5 | Apatite | WHEEL | With a big appetite for running |
| 6 | Iridium | SASH | Irritating the wearer |
| 7 | Quartz | EGG | Quartz taken from an Egg |
| 8 | Topaz | IVY | To pass the Ivy |
| 9 | Corundum | WHIP | Made Cora run |
| 10 | Diamond | TOES | Set with Diamonds |
In the list you have ten substances. Kaolin, the base, is 1, Rock Salt is 2, which indicates that Rock Salt is twice as hard as Kaolin. Iridium is 6, and six times as hard as Kaolin. Diamond is 10, which means that it is ten times as hard as Kaolin.
Learning this list by picturing reminders with the word of the Code list will enable you to easily recall these ten substances and the degree of hardness compared with Kaolin.
Atomic Weight Table
Some students of Chemistry have learned the entire list of elements and their atomic weights. The following are a few examples of how the list can be arranged and learned. It will be excellent practice for you to use this method and make a list of your own.
| Element | Code No. | Reminder | Wt. | Code Word | |
| 1 | Carbon | TIE | Carbine | 12.005 | Tin Sizzle |
| 2 | Hydrogen | SNOW | Hydrant | 1.008 | The Saucy Foe |
| 3 | Nitrogen | HOME | Night | 14.01 | Deer Sat |
| 4 | Oxygen | WIRE | Ox | 16.0 | Dash |
| 5 | Sulphur | WHEEL | Sulphur | 32.06 | Money Sash |
| 6 | Phosphorus | SASH | Fuss for us | 31.04 | Mad Sir |
| 7 | Sodium | EGG | Soda | 23.0 | Nome |
| 8 | Potassium | IVY | Pot | 39.1 | Mop It |
| 9 | Calcium | WHIP | Calsomine | 40.07 | Horse Sack |
| 10 | Iron | TOES | I Run | 55.84 | Lily Fire |
| 11 | Arsenic | DOT | Arson | 74.96 | Gray Page |
| 12 | Gold | TOWN | Gold | 197.2 | Dipping In |