Availability
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Overview of available volumes.
| Volume | Year | Letters | PG ebook | Scans |
| I | 1871 | I–XII | [59456] | 1 |
| II | 1872 | XII–XXIV | [61591] | 2 |
| III | 1873 | XXV–XXXVI | [61634] | 3 |
| IV | 1874 | XXXVII–XLVIII | [67544] | 4 |
| V | 1875 | XLIX–LX | [68013] | 5 |
| VI | 1876 | LXI–LXXII | This volume | 6 |
| VII | 1877 | LXXIII–LXXXIV | In preparation | 7 |
| VIII | 1878–’84 | LXXXV–XCVI | In preparation | 8 |